kbuild: fix the warning when running make tags

make tags was giving the below warning.

ctags: Warning: arch/x86_64/kernel/head.S:124: null expansion of name
pattern "\1"

Fix the same by making sure we taken only ENTRY pattern found at the
begining of the line.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>