kernel/lib: add missing musl include

This is needed to fix the build on arm as atomic_arch.h
relies on musl libc.h and features.h

Change-Id: I5bfa32b7b0fa1662cc9ee8cd8fa3037d8a5e2431
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5418
Reviewed-by: Adrien Destugues <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/system/kernel/lib/Jamfile

@@ -52,6 +52,7 @@ local muslSources =
 	;
 
 SourceHdrs $(muslSources) :
+	[ FDirName $(posixSources) musl include ]
 	[ FDirName $(posixSources) musl internal ]
 	[ FDirName $(posixSources) musl arch $(TARGET_KERNEL_ARCH_DIR) ] ;