meson.build: turn on -Wall

Doesn't seem to add any warnings right now. -Wextra does, though, and
there's a ton of them. Need to run through those and see if there's
anything we care about.

Signed-off-by: Steven Noonan <steven@valvesoftware.com>

src/meson.build

@@ -30,7 +30,7 @@ flags_common = [
 ]
 
 warn_flags_common = [
-  #'-Wall',
+  '-Wall',
   #'-Wextra',
   #'-pedantic',
   '-Wno-unknown-pragmas',