hdata/tests/stubs.c: fix GCC8 warning

hdata/test/stubs.c:112:11: error: ‘lock_caller’ alias between functions of incompatible types ‘void(void)’ and ‘_Bool(void)’ [-Werror=attribute-alias]
 NOOP_STUB(lock_caller);
           ^~~~~~~~~~~

We fix it by giving the correct prototype to our stub

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

hdata/test/stubs.c

@@ -103,9 +103,9 @@ static bool true_stub(void) { return true; }
 static bool false_stub(void) { return false; }
 
 #define TRUE_STUB(fnname) \
-	void fnname(void) __attribute__((weak, alias ("true_stub")))
+	bool fnname(void) __attribute__((weak, alias ("true_stub")))
 #define FALSE_STUB(fnname) \
-	void fnname(void) __attribute__((weak, alias ("false_stub")))
+	bool fnname(void) __attribute__((weak, alias ("false_stub")))
 #define NOOP_STUB FALSE_STUB
 
 TRUE_STUB(lock_held_by_me);