exec: Fix broken build for MinGW (regression)

Commit 3435f395 reduced the ifdeffery with
this result for MinGW:

exec.c: In function ‘qemu_ram_free’:
exec.c:1239:17: warning:
 implicit declaration of function ‘munmap’ [-Wimplicit-function-declaration]
exec.c:1239:17: warning:
 nested extern declaration of ‘munmap’ [-Wnested-externs]
exec.c:1239: undefined reference to `munmap'

Add some ifdeffery again to fix this.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>