kbuild: clear KBUILD_SRC when calling 'make' in RPM spec

'make rpm-pkg' and 'make binrpm-pkg' fail when the kernel source is
read-only.  Specifically, when the RPM spec generated by
scripts/package/mkspec is run, KBUILD_SRC happens to be set to the
source location and thus the invocation of 'make headers_install'
fails when an internal call to 'filechk' tries to write a file into
the source tree.

The fix is to clear KBUILD_SRC for the 'make headers_install'
invocation in the spec file, as is already done for the 'make
modules_install' invocation.

Signed-off-by: David R. Bild <drbild@umich.edu>
Signed-off-by: Michal Marek <mmarek@suse.cz>