Use -Werror in external/selinux

* Use -Wno-error= to keep existing warnings to fix later.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I0e84d5fb2ae2ae68a687a0b6b81eb64a983db57c

checkpolicy/Android.bp

@@ -21,6 +21,7 @@ cc_binary_host {
 
 cc_binary_host {
     name: "dispol",
+    cflags: common_CFLAGS,
     srcs: ["test/dispol.c"],
     static_libs: ["libsepol"],
 }

libselinux/Android.bp

@@ -9,6 +9,11 @@ common_CFLAGS = [
     "-DNO_MEDIA_BACKEND",
     "-DNO_X_BACKEND",
     "-DNO_DB_BACKEND",
+    "-Wall",
+    "-Werror",
+    "-Wno-error=missing-noreturn",
+    "-Wno-error=unused-function",
+    "-Wno-error=unused-variable",
 ]
 
 cc_defaults {

libsepol/Android.bp

@@ -1,10 +1,11 @@
 common_CFLAGS = [
     "-D_GNU_SOURCE",
     "-Wall",
+    "-Werror",
     "-W",
     "-Wundef",
     "-Wshadow",
-    "-Wmissing-noreturn",
+    "-Wno-error=missing-noreturn",
     "-Wmissing-format-attribute",
 ]