third_party/googletest: Disable -Wconversion

GoogleTest does not compile with -Wconversion, so disable it.

Example error:

  ../third_party/googletest/googletest/include/gtest/gtest-printers.h:531:9: error: implicit conversion from 'int32_t' (aka 'int') to 'float' may lose precision [-Werror,-Wimplicit-int-float-conversion]
      if (static_cast<int32_t>(val * mulfor6 + 0.5) / mulfor6 == val) return 6;

Bug: b/262281394, b/259746255
Change-Id: Ib775b1081b7132c31bfb08fa5593cb5dfa719cd7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/124910
Reviewed-by: Sergio Soares <[email protected]>
Pigweed-Auto-Submit: Wyatt Hepler <[email protected]>
Reviewed-by: Alexei Frolov <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

third_party/googletest/BUILD.gn

@@ -32,7 +32,10 @@ if (dir_pw_third_party_googletest != "") {
     ]
 
     # Fix some compiler warnings.
-    cflags = [ "-Wno-undef" ]
+    cflags = [
+      "-Wno-undef",
+      "-Wno-conversion",
+    ]
   }
 
   pw_source_set("googletest") {