[libFuzzer] Use full path to libfuzzer_exports.h in fuzzer_test.gni.

TBR=rsesek@chromium.org

Bug: 687076
Change-Id: Ia3e3f412fe8fa61c19887573e01cffd0ed751bff
Reviewed-on: https://chromium-review.googlesource.com/724729Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509650}

testing/libfuzzer/fuzzer_test.gni

@@ -138,7 +138,7 @@ template("fuzzer_test") {
       }
 
       if (is_mac) {
-        sources += [ "libfuzzer_exports.h" ]
+        sources += [ "//testing/libfuzzer/libfuzzer_exports.h" ]
       }
     }
   } else {