devtools: fix symbol check when adding experimental section

devtools: fix symbol check when adding experimental section

The incriminated commit did relax the condition to catch all sections
but dropped the + removal which can trigger false detection of the
special EXPERIMENTAL section when adding symbols and the section in the
same patch.

Fixes: 7281cf520f89 ("devtools: relax rule for identifying symbol section")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>