bpf: correct extra_headers

The BPF CO-RE support (commit 8bdabb3754)
mistakenly overwrote bpf-*-* extra_headers in config.gcc, causing
bpf-helpers.h to not be installed. The redefinition with coreout.h is
unneeded, so delete it.

gcc/ChangeLog:

	* config.gcc (bpf-*-*): Do not overwrite extra_headers.

gcc/config.gcc

@@ -1531,7 +1531,6 @@ bpf-*-*)
         use_collect2=no
         extra_headers="bpf-helpers.h"
         use_gcc_stdint=provide
-        extra_headers="coreout.h"
         extra_objs="coreout.o"
         target_gtfiles="$target_gtfiles \$(srcdir)/config/bpf/coreout.c"
         ;;