[gdb/testsuite] Fix regexp in gdb.dwarf2/dw2-inter-cu-error.exp

In commit b5070480d74 ("[gdb/symtab] Change DWARF_ERROR from Dwarf Error to
DWARF Error") I changed the dwarf error prefix, but failed to update test-case
gdb.dwarf2/dw2-inter-cu-error.exp.

Fix this by updating the corresponding regexp in the test-case.

Tested on x86_64-linux.
1 file changed