rp-pppoe: discard use of dnl in Makefile.am

Since Makefile.am is not processed by m4, but by automake, 'dnl' does
not introduce a comment.

Fix the following error: (appearred on opensuse-11.4/fedora-15, which
shipped make with version 3.82)
...
NOTE: package rp-pppoe-3.8-r8: task do_compile: Started
...
| NOTE: make -j 3
| Makefile:640: *** missing separator.  Stop.
| ERROR: oe_runmake failed
NOTE: package rp-pppoe-3.8-r8: task do_compile: Failed

Signed-off-by: Ting Liu <b28495@freescale.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>