perfetto: Fix build flag to detect gcc

- Define the variable is_gcc to substitute (!is_clang && !is_win)

Change-Id: I15723df8192d3dddd112b9c64e7371ee03674a97

gn/standalone/BUILD.gn

diff --git a/gn/standalone/BUILD.gn b/gn/standalone/BUILD.gn
index ecd5c95..36fb637 100644
--- a/gn/standalone/BUILD.gn
+++ b/gn/standalone/BUILD.gn
@@ -95,7 +95,7 @@
       # codebase cleanup.
       "-Wno-switch-default",
     ]
-  } else if (!is_clang && !is_win) {
+  } else if (is_gcc) {
     # Use return std::move(...) for compatibility with old GCC compilers.
     cflags_cc = [ "-Wno-redundant-move" ]