build: removed undefined behavior from PVR check

This shouldn't matter, but it's probably better if the code to check is
valid - otherwise an extremely clever compiler might fail to compile it,
and the feature would be misdetected. (Probably.)

Found by cppcheck.

waftools/fragments/pvr.c

@@ -3,5 +3,5 @@
 int main(void)
 {
     struct v4l2_ext_controls ext;
-    return ext.controls->value;
+    return !!&ext.controls->value;
 }