Disable -Wimplicit-fallthrough for now

Unfortunately with clang10 this generates lots of spurious warnings from
SDL headers, even though they have fallthrough comments.

meson.build

@@ -91,11 +91,11 @@ taisei_c_args = [
     '-Wgcc-compat',
     '-Wgnu',
     '-Wignored-qualifiers',
-    '-Wimplicit-fallthrough',
     '-Winit-self',
     '-Wlogical-op',
     '-Wmissing-prototypes',
     '-Wno-gnu-folding-constant',
+    '-Wno-implicit-fallthrough',
     '-Wno-long-long',
     '-Wno-missing-braces',
     '-Wno-typedef-redefinition',