fuzz: always define LLVMFuzzerInitialize() even if we don't need it

This will make it easier to test our fuzzers outside oss-fuzz.

Differential Revision: https://phab.mercurial-scm.org/D7560

contrib/fuzz/bdiff.cc

@@ -14,6 +14,11 b''
 extern "C" {
 #include "bdiff.h"
 
+extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	return 0;
+}
+
 int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size)
 {
 	FuzzedDataProvider provider(Data, Size);

contrib/fuzz/mpatch.cc

@@ -14,6 +14,11 b''
 
 #include "fuzzutil.h"
 
+extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	return 0;
+}
+
 // To avoid having too many OOMs from the fuzzer infrastructure, we'll
 // skip patch application if the resulting fulltext would be bigger
 // than 10MiB.

contrib/fuzz/xdiff.cc

@@ -14,6 +14,11 b''
 
 extern "C" {
 
+int LLVMFuzzerInitialize(int *argc, char ***argv)
+{
+	return 0;
+}
+
 int hunk_consumer(long a1, long a2, long b1, long b2, void *priv)
 {
 	// TODO: probably also test returning -1 from this when things break?