GN: Fix errors in rtc_include_tests conditions

https://codereview.webrtc.org/2043873003 forgot to flip the
logic.

BUG=webrtc:5949
TBR=phoglund@webrtc.org
NOTRY=True

Review URL: https://codereview.webrtc.org/2037403003 .

Cr-Commit-Position: refs/heads/master@{#13055}

webrtc/base/BUILD.gn

@@ -632,7 +632,7 @@ source_set("gtest_prod") {
   ]
 }
 
-if (!rtc_include_tests) {
+if (rtc_include_tests) {
   config("rtc_base_test_utils_exported_config") {
     defines = [ "GTEST_RELATIVE_PATH" ]
   }