Fix gdb/spu-tdep.c build breakage

Fix gdb/spu-tdep.c build breakage

Commit de63c46b549d ("Fix regresssion(internal-error) printing
subprogram argument (PR gdb/22670)") missed updating spu-tdep.c for
the block_lookup_symbol interface change, resulting in:

  ../../binutils-gdb/gdb/spu-tdep.c: In function void spu_catch_start(objfile*):
  ../../binutils-gdb/gdb/spu-tdep.c:1969:59: error: cannot convert domain_enum_tag to symbol_name_match_type for argument 3 to symbol* block_lookup_symbol(const block*, const char*, symbol_name_match_type, domain_enum)
	 sym = block_lookup_symbol (block, "main", VAR_DOMAIN);
							     ^

gdb/ChangeLog:
2018-01-05  Pedro Alves  <palves@redhat.com>

	* spu-tdep.c (spu_catch_start): Pass
	symbol_name_match_type::SEARCH_NAME to block_lookup_symbol.