[build] Set -Wno-strict-prototypes.

https://reviews.llvm.org/D122895 changed the semantics of
-Wstrict-prototypes, which causes -Werror build failures in the core
builders. To fix the warning, this series of changes sets
-Wno-strict-prototypes on affected third-party dependencies.

The error message is "a function declaration without a prototype is
deprecated in all versions of C". It appears in:
  trees.c
  contrib/minizip/ioapi.c
  contrib/minizip/zip.c

Bug: 97746
Change-Id: If13e83d530147fe78956a94708c3f57e803c1ed8
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/zlib/+/668884
Reviewed-by: Gulfem Savrun Yeniceri <gulfem@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 205fa7e..390daae 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -37,6 +37,7 @@
     "-Wno-implicit-function-declaration",
     "-Wno-conversion",
     "-Wno-implicit-fallthrough",
+    "-Wno-strict-prototypes",
   ]
 }
 
@@ -132,6 +133,7 @@
       "-Wno-misleading-indentation",
       "-Wno-conversion",
       "-Wno-unused-but-set-variable",
+      "-Wno-strict-prototypes",
     ]
   }
 }