This adjusts gdb.python/py-nested-maps.c to make it buildable as C++ program.
key_t is renamed because of:
src/gdb/testsuite/gdb.python/py-nested-maps.c:23:8: error: definition of type 'key_t' conflicts with typedef of the same name
struct key_t
^
/usr/include/x86_64-linux-gnu/sys/types.h:121:17: note: 'key_t' declared here
typedef __key_t key_t;
^