w32: Fix missing declaration of ffs()

target-ppc/cpu.h now needs ffs(), too, so ffs() must be declared
before this file is included.

Moving the declaration from qemu-common.h to qemu-os-win32.h
(which is included in qemu-common.h early) fixes the compiler
warning for w32.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
diff --git a/qemu-os-win32.h b/qemu-os-win32.h
index ed2753d..8a069d7 100644
--- a/qemu-os-win32.h
+++ b/qemu-os-win32.h
@@ -29,6 +29,9 @@
 #include <windows.h>
 #include <winsock2.h>
 
+/* Declaration of ffs() is missing in MinGW's strings.h. */
+int ffs(int i);
+
 /* Polling handling */
 
 /* return TRUE if no sleep should be done afterwards */