fuzzer/*: normalize src/ includes

this uses the format introduced to some files in:
cc3577e9 fuzzer/*: use src/ based include paths

Change-Id: I9b5cbeadbb9d54d1e89f474a6e479a5eb3175ed7

tests/fuzzer/advanced_api_fuzzer.c

@@ -17,7 +17,7 @@
 #include <string.h>
 
 #include "./fuzz_utils.h"
-#include "webp/decode.h"
+#include "src/webp/decode.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t* const data, size_t size) {
   int i;