build: Enable Wextra with some exceptions

-Wno-cast-function-type:
Signal handler code relies on this.

-Wno-type-limits:
Some of the color space conversion code in zm_image throws this warning. To not break anything let's ignore this warning for the time being.

Note that clang enables -Woverloaded-virtual with -Wextra. To not flood the GCC build log with warnings let's not enable this at the moment.
This will be done in a second step, when the currently throwing code will be cleaned up.

cmake/compiler/clang/settings.cmake

@@ -1,6 +1,8 @@
 target_compile_options(zm-warning-interface
   INTERFACE
-    -Wall)
+    -Wall
+    -Wextra
+    -Wno-unused-parameter)
 
 if(ASAN)
   target_compile_options(zm-compile-option-interface

cmake/compiler/gcc/settings.cmake

@@ -1,6 +1,10 @@
 target_compile_options(zm-warning-interface
   INTERFACE
-    -Wall)
+    -Wall
+    -Wextra
+    -Wno-cast-function-type
+    -Wno-type-limits
+    -Wno-unused-parameter)
 
 if(ASAN)
   target_compile_options(zm-compile-option-interface