[compile] Suppress unused variables warning

Bug: 77973
Change-Id: I1dec9813a0c6412fb484c3d6e0bd665e25f39b85
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/538888
Reviewed-by: Petr Hosek <phosek@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index cb6b76d..274c4aa 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -21,6 +21,9 @@
       # On platforms where size_t is 64-bits, this comparison will always
       # be false.
       "-Wno-tautological-constant-out-of-range-compare",
+
+      "-Wno-unused-but-set-parameter",
+      "-Wno-unused-but-set-variable",
     ]
   }
 }