kbuild: use POSIX BRE in headers install target

The sed expression used at the moment in scripts/Makefile.headersinst
relies on the (handy) GNU extension where you can escape ERE's in an
otherwise BRE without using the GNU -r option.  The following patch
replaces this "\+" usage with a functionally equivalent POSIX BRE compliant
"\{1,\}".  Tested with `make headers_install` against blackfin/x86_64/i386
targets.

Stupid whiny OS X users and their crappy sed ;)

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>