make $if work with includes: skip the body if the target is different

vlib/runtime/runtime.v

@@ -1 +1,14 @@
 module runtime
+
+$if linux {
+	#include <sys/sysinfo.h>
+	fn C.get_nprocs() int
+}
+
+
+pub fn nr_cpus() int {
+	$if linux {
+		return C.get_nprocs()
+	}
+	return 0
+}