build: fix mingw build issues

8901fca027 broke the build under mingw, this
fixes that.

Change-Id: I22b91e220dac3b68cc576b65a9f1b8711e64263a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/298
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/helper/replacements.h

@@ -161,6 +161,9 @@ static __inline void outb(unsigned char value, unsigned short int port)
 	__asm__ __volatile__ ("outb %b0,%w1": :"a" (value), "Nd" (port));
 }
 
+/* mingw does not have ffs, so use gcc builtin types */
+#define ffs __builtin_ffs
+
 #endif /* IS_MINGW */
 
 int win_select(int max_fd, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *tv);