coverage: fix build with g++

coverage: fix build with g++

Guard the coverage-related shenanigans from g++ when building the one
C++ unit test we have, so we don't have to make it C++ compatible:

[1573/2109] Compiling C++ object test-bus-vtable-cc.p/src_libsystemd_sd-bus_test-bus-vtable-cc.cc.o
FAILED: test-bus-vtable-cc.p/src_libsystemd_sd-bus_test-bus-vtable-cc.cc.o
ccache c++ -Itest-bus-vtable-cc. ... -c ../src/libsystemd/sd-bus/test-bus-vtable-cc.cc
In file included from <command-line>:
../src/basic/coverage.h:17:15: error: ‘_Noreturn’ does not name a type
   17 | static inline _Noreturn void _coverage__exit(int status) {
      |               ^~~~~~~~~
../src/basic/coverage.h:46:29: error: conflicting declaration of ‘int _coverage_execveat(int, const char*, char* const*, char* const*, int)’ with ‘C’ linkage
   46 | #define execveat(d,p,a,e,f) _coverage_execveat(d, p, a, e, f)
      |                             ^~~~~~~~~~~~~~~~~~
../src/basic/coverage.h:34:19: note: previous declaration with ‘C++’ linkage
   34 | static inline int _coverage_execveat(
      |                   ^~~~~~~~~~~~~~~~~~
../src/basic/coverage.h:46:29: error: declaration of ‘int _coverage_execveat(int, const char*, char* const*, char* const*, int) noexcept’ has a different exception specifier
   46 | #define execveat(d,p,a,e,f) _coverage_execveat(d, p, a, e, f)
      |                             ^~~~~~~~~~~~~~~~~~
../src/basic/coverage.h:34:19: note: from previous declaration ‘int _coverage_execveat(int, const char*, char* const*, char* const*, int)’
   34 | static inline int _coverage_execveat(
      |                   ^~~~~~~~~~~~~~~~~~
../src/basic/coverage.h:58:24: error: conflicting declaration of ‘int _coverage_execvpe(const char*, char* const*, char* const*)’ with ‘C’ linkage
   58 | #define execvpe(f,a,e) _coverage_execvpe(f, a, e)
      |                        ^~~~~~~~~~~~~~~~~
../src/basic/coverage.h:48:19: note: previous declaration with ‘C++’ linkage
   48 | static inline int _coverage_execvpe(
      |                   ^~~~~~~~~~~~~~~~~
../src/basic/coverage.h:58:24: error: declaration of ‘int _coverage_execvpe(const char*, char* const*, char* const*) noexcept’ has a different exception specifier
   58 | #define execvpe(f,a,e) _coverage_execvpe(f, a, e)
      |                        ^~~~~~~~~~~~~~~~~
../src/basic/coverage.h:48:19: note: from previous declaration ‘int _coverage_execvpe(const char*, char* const*, char* const*)’
   48 | static inline int _coverage_execvpe(
      |                   ^~~~~~~~~~~~~~~~~
[1582/2109] Compiling C object test-event.p/src_libsystemd_sd-event_test-event.c.o
ninja: build stopped: subcommand failed.