UBUNTU: [debian] prep linux-libc-dev only if do_libc_dev_package=true

Required for do_libc_dev_package=false to not break the build.

BugLink: http://bugs.launchpad.net/bugs/1764794Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

debian/rules.d/2-binary-arch.mk

@@ -411,7 +411,9 @@ install-arch-headers:
 	@echo Debug: $@
 	dh_testdir
 	dh_testroot
+ifeq ($(do_libc_dev_package),true)
 	dh_prep -plinux-libc-dev
+endif
 
 	rm -rf $(headers_tmp)
 	install -d $(headers_tmp) $(headers_dir)/usr/include/