Makefile: ensure build-head works out-of-src-tree

Pass -C $(SRCDIR) to the git command that generates the build-head
stamp.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>