[klibc] Fix header installation from out-of-tree build

[klibc] Fix header installation from out-of-tree build

Origin: https://git.kernel.org/pub/scm/libs/klibc/klibc.git/commit?id=8d6ee65cc8bd75bba8f51589c667ff7c5fd9b3b5
Bug-Debian: https://bugs.debian.org/1004465

In an out-of-tree build we currently only install the kernel UAPI
headers and the generated <klibc/havesyscall.h>.  Add an extra
command to copy headers from the source tree in an out-of-tree
build.

References: https://bugs.debian.org/1004465
Reported-by: Thorsten Glaser <tg@mirbsd.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Name 0001-klibc-Fix-header-installation-from-out-of-tree-build.patch