uxpgnt --askpass: explicitly fflush(stdout) on exit.

I'm not really sure why that's necessary: by my understanding of the C
standard, it shouldn't be. But my observation is that when compiling
with {Address,Leak} Sanitiser enabled, pageant --askpass can somehow
manage to exit without having actually written the passphrase to its
standard output.

(cherry picked from commit c618d6baac)

unix/uxpgnt.c

@@ -1093,6 +1093,8 @@ int main(int argc, char **argv)
             return 1;
 
         puts(passphrase);
+        fflush(stdout);
+
         smemclr(passphrase, strlen(passphrase));
         sfree(passphrase);
         return 0;