bootgraph: fix for use with dot symbols

powerpc has dot symbols, so the dmesg output looks like:

<4>[    0.327310] calling  .migration_init+0x0/0x9c @ 1
<4>[    0.327595] initcall .migration_init+0x0/0x9c returned 1 after 0 usecs

The below fixes bootgraph.pl so it handles this correctly.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>