cmd/gc: fix build

Darwin 10.6 (gcc 4.2) and some older versions of gcc default to C90 mode, not C99 mode. Silence the warning.

LGTM=aram, iant
R=golang-codereviews, aram, iant
CC=golang-codereviews
https://golang.org/cl/83090050

src/cmd/gc/plive.c

@@ -1633,8 +1633,8 @@ livenessepilogue(Liveness *lv)
 }
 
 // FNV-1 hash function constants.
-#define H0 2166136261
-#define Hp 16777619
+#define H0 2166136261UL
+#define Hp 16777619UL
 
 static uint32
 hashbitmap(uint32 h, Bvec *bv)