[gdb/testsuite] Fix -readnow FAIL in gdb.base/style.exp

When running test-case gdb.base/style.exp with target board readnow, we run
into:
...
FAIL: gdb.base/style.exp: filename is styled when loading symbol file
...

The problem is that with -readnow, an extra "Expanding full symbols" message
is generated:
...
(gdb) file $file^M
Reading symbols from $file...^M
Expanding full symbols from $file...^M
(gdb) FAIL: gdb.base/style.exp: filename is styled when loading symbol file
...
and the test does not expect this message.

Fix this by expecting the additional message for -readnow.

gdb/testsuite/ChangeLog:

2020-04-10  Tom de Vries  <tdevries@suse.de>

	* gdb.base/style.exp: Expect "Expanding full symbols" message for
	-readnow.