compat/mingw.h: drop extern from function declaration

In 5545442 (*.[ch]: remove extern from function declarations using
spatch, 2019-04-29), `extern` on function declarations were declared to
be redundant and thus removed from the codebase. An `extern` was
accidentally reintroduced in 08809c0 (mingw: add a helper function to
attach GDB to the current process, 2020-02-13).

Remove this spurious `extern`.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

compat/mingw.h

@@ -606,7 +606,7 @@ int main(int argc, const char **argv);
  * Call this function to open a new MinTTY (this assumes you are in Git for
  * Windows' SDK) with a GDB that attaches to the current process right away.
  */
-extern void open_in_gdb(void);
+void open_in_gdb(void);
 
 /*
  * Used by Pthread API implementation for Windows