fuzz: Fix interfacing with LLVMFuzzer functions (#2068)

tests/fuzzing/synth_fuzzer.c

 /* See http://llvm.org/docs/LibFuzzer.html */
 extern int LLVMFuzzerRunDriver(int *argc, char ***argv,
                   int (*UserCb)(const uint8_t *Data, size_t Size));
+extern int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {

tests/fuzzrunner.c

 
 #include <espeak-ng/espeak_ng.h>
 
-int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+extern int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
 
 int main(int argc, char **argv) {
 	int i;

tests/ssml-fuzzer.c

 
 char *filepath = NULL;
 
-extern int LLVMFuzzerInitialize(const int* argc, char*** argv)
-{
+int LLVMFuzzerInitialize(const int* argc, char*** argv) {
 	(void)argc; // unused
 	filepath = dirname(strdup((*argv)[0]));
 	return 0;
 }
 
-extern int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 	if (!initialized) {
 		const char *hasDataPath = getenv("ESPEAK_DATA_PATH");
 		if (!hasDataPath) {