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

This reverts commit ec535cc27e6c4f5e0b1d157e04f5511f166ecd9d.

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>