re-add useconds_t

this type was removed back in 5243e5f160 ,
because it was removed from the XSI specs.
however some apps use it.
since it's in the POSIX reserved namespace, we can expose it
unconditionally.

arch/microblaze/bits/alltypes.h.sh

@@ -61,6 +61,7 @@ TYPEDEF double double_t;
 
 TYPEDEF long time_t;
 TYPEDEF int suseconds_t;
+TYPEDEF unsigned useconds_t;
 STRUCT timeval { time_t tv_sec; int tv_usec; };
 STRUCT timespec { time_t tv_sec; long tv_nsec; };