Revert "disable -Wcast-qual because of musl"

This reverts commit 05780b63.

If we have to choose between this warning and alpine being part of our
CI, this warning takes precedence. It's always possible to disable
developer warnings on CCI if that's a problem with musl then we can
configure its CCI job to disable developer warnings.

Reopens #3565
Refs #3568

wflags.py

@@ -44,6 +44,7 @@ DESIRABLE_OPTIONS = [
 
 DESIRABLE_WFLAGS = [
     "-Wcast-align",
+    "-Wcast-qual",
     "-Wchar-subscripts",
     "-Wempty-body",
     "-Wextra",
@@ -68,7 +69,6 @@ DESIRABLE_WFLAGS = [
 ]
 
 UNDESIRABLE_WFLAGS = [
-    "-Wno-cast-qual", # GCC complains about musl::sched.h
     "-Wno-thread-safety", # Does not understand our mutexs are wrapped
     "-Wno-old-style-definition", # Does not like vgz
     "-Wno-sign-compare", # Fixable