[gdb/testsuite] Fix gdb.dwarf2/nullptr_t.exp with cc-with-dwz-m

[gdb/testsuite] Fix gdb.dwarf2/nullptr_t.exp with cc-with-dwz-m

When running test-case gdb.dwarf2/nullptr_t.exp with target board
cc-with-dwz-m, I run into:
...
FAIL: gdb.dwarf2/nullptr_t.exp: decltype(nullptr) symbol
...

The problem is that were looking for "typedef void decltype\\(nullptr\\)"
using "maint print symbols -source $srcfile", but dwz has moved the typedef to
a PU, so it's shown by "maint print symbols -source <unknown>" instead.

Fix this by dropping the "-source $srcfile" bit.

Tested on x86_64-linux, with make-check-all.sh.