Revert "compat/unsetenv.c: Fix a sparse warning"

This reverts commit ec535cc27e.

POSIX explicitly states "the [environ] variable, which
must be declared by the user if it is to be used directly".
Not declaring it causes compilation to fail on OS X.

Instead don't declare the variable on MinGW, as it causes
a spurious warning there.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/unsetenv.c

@@ -2,6 +2,9 @@
 
 void gitunsetenv (const char *name)
 {
+#if !defined(__MINGW32__)
+     extern char **environ;
+#endif
      int src, dst;
      size_t nmln;