[fuchsia] Disable implicit int float conversion warning.

This will only affect targets building with gn.

Change-Id: I8a0ed533d0b224bfebdfd41925fcdcad11b34717

BUILD.gn

@@ -29,6 +29,6 @@ config("abseil_config") {
     # TODO: disable -Wimplicit-int-float-conversion which has been introduced
     # in https://reviews.llvm.org/D64666 and is failing in duration.cc:190:12
     "-Wno-unknown-warning-option",
-    "-Wno-error=implicit-int-float-conversion",
+    "-Wno-implicit-int-float-conversion",
   ]
 }