musl: fixed to also force reinstall of arch specific headers

This fixes the issue that arch specific headers will not appear in the flist
file when cross-compiling, and thus being missing in the resulting binary
package.

base/musl/musl.conf

@@ -57,5 +57,5 @@ if [ "$SDECFG_LIBC" == "musl" ]; then
 fi
 
 # always install the headers even if already present in the sandbox
-hook_add postpatch 9 "touch include/*.h include/*/*.h"
+hook_add postpatch 9 "touch include/*.h include/*/*.h arch/$musl_arch/*.h arch/$musl_arch/*/*.h"
 var_append makeinstopt ' ' "install-headers"