openjfx11: fix build

Remove -Werror=implicit-function-declaration, which when passed to cmake
for webkit build breaks C++ compiler configuration due to spurious warnings:
`cc1plus: warning: '-Werror=' argument '-Werror=implicit-function-declaration' is not valid for C++`

pkgs/development/compilers/openjdk/openjfx/11.nix

@@ -24,6 +24,11 @@ let
 
     dontUseCmakeConfigure = true;
 
+    postPatch = ''
+      substituteInPlace buildSrc/linux.gradle \
+        --replace ', "-Werror=implicit-function-declaration"' ""
+    '';
+
     config = writeText "gradle.properties" (''
       CONF = Release
       JDK_HOME = ${openjdk11-bootstrap.home}