gdbsupport/common-inferior.c: Fix mingw build

gdbsupport/common-inferior.c: Fix mingw build

A recent change (512ca2fca4b "gdb: split up
construct_inferior_arguments") introduced a build failure for mingw:

      CXX      common-inferior.o
    .../gdb/gdbsupport/common-inferior.cc: In function ‘std::string escape_characters(const char*, const char*)’:
    .../gdb/gdbsupport/common-inferior.cc:62:20: error: ‘argv’ was not declared in this scope; did you mean ‘arg’?
       62 |       if (strpbrk (argv[i], special))
          |                    ^~~~
          |                    arg
    .../gdb/gdbsupport/common-inferior.cc:62:25: error: ‘i’ was not declared in this scope
       62 |       if (strpbrk (argv[i], special))
          |                         ^

This patch fixes that.

Change-Id: I07ade607bc4516627b433085b07d9d198d8e4b4a
Approved-By: Tom de Vries <tdevries@suse.de>