mingw: drop bogus (and unneeded) declaration of `_pgmptr`

mingw: drop bogus (and unneeded) declaration of `_pgmptr`

In 08809c09aa13 (mingw: add a helper function to attach GDB to the
current process, 2020-02-13), I added a declaration that was not needed.
Back then, that did not matter, but now that the declaration of that
symbol was changed in mingw-w64's headers, it causes the following
compile error:

      CC compat/mingw.o
  compat/mingw.c: In function 'open_in_gdb':
  compat/mingw.c:35:9: error: function declaration isn't a prototype [-Werror=strict-prototypes]
     35 |         extern char *_pgmptr;
        |         ^~~~~~
  In file included from C:/git-sdk-64/usr/src/git/build-installers/mingw64/lib/gcc/x86_64-w64-mingw32/14.1.0/include/mm_malloc.h:27,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/lib/gcc/x86_64-w64-mingw32/14.1.0/include/xmmintrin.h:34,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/lib/gcc/x86_64-w64-mingw32/14.1.0/include/immintrin.h:31,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/lib/gcc/x86_64-w64-mingw32/14.1.0/include/x86intrin.h:32,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/include/winnt.h:1658,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/include/minwindef.h:163,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/include/windef.h:9,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/include/windows.h:69,
                   from C:/git-sdk-64/usr/src/git/build-installers/mingw64/include/winsock2.h:23,
                   from compat/../git-compat-util.h:215,
                   from compat/mingw.c:1:
  compat/mingw.c:35:22: error: '__p__pgmptr' redeclared without dllimport attribute: previous dllimport ignored [-Werror=attributes]
     35 |         extern char *_pgmptr;
        |                      ^~~~~~~

Let's just drop the declaration and get rid of this compile error.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>