util: hash: Make virHashCodeGen mockable

Export the function from the util module so that dynamic linking can
override it.

src/libvirt_private.syms

@@ -1774,6 +1774,10 @@ virHashUpdateEntry;
 virHashValueFree;
 
 
+# util/virhashcode.h
+virHashCodeGen;
+
+
 # util/virhook.h
 virHookCall;
 virHookInitialize;

src/util/virhashcode.h

@@ -30,6 +30,7 @@
 
 # include "internal.h"
 
-uint32_t virHashCodeGen(const void *key, size_t len, uint32_t seed);
+uint32_t virHashCodeGen(const void *key, size_t len, uint32_t seed)
+    ATTRIBUTE_NOINLINE;
 
 #endif /* __VIR_HASH_CODE_H__ */