[build] Suppress -Wconversion warnings

Allows downstream projects that specify -Wconversion to build goldfish-opengl.

Bug: fuchsia:58162
Change-Id: I73e5cf39e578c5b98ea4cbe27b81e845690228ea
diff --git a/BUILD.gn b/BUILD.gn
index 9941c5f..76bf904 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -116,6 +116,7 @@
     "-Wno-unused-function",
     "-Wno-unused-value",
     "-Wno-unused-variable",
+    "-Wno-conversion",
   ]
 
   ldflags = [ "-static-libstdc++" ]