Fix lrealpath memory leak in build_id_to_debug_bfd

Fix lrealpath memory leak in build_id_to_debug_bfd

Valgrind reports memory leaking from build_id_to_debug_bfd:
==7261== 88 bytes in 2 blocks are definitely lost in loss record 31,319 of 35,132
==7261==    at 0x4A06BCF: malloc (vg_replace_malloc.c:296)
==7261==    by 0x32CA88A9B9: strdup (strdup.c:42)
==7261==    by 0xFE62AB: lrealpath (lrealpath.c:88)
==7261==    by 0x7F7AD6: build_id_to_debug_bfd (build-id.c:116)
==7261==    by 0x7F7BB5: find_separate_debug_file_by_buildid (build-id.c:149)
==7261==    by 0x6D9382: elf_symfile_read (elfread.c:1348)
==7261==    by 0x777F02: read_symbols (symfile.c:875)
==7261==    by 0x778505: syms_from_objfile_1 (symfile.c:1078)
==7261==    by 0x778548: syms_from_objfile (symfile.c:1094)
==7261==    by 0x778746: symbol_file_add_with_addrs (symfile.c:1191)
==7261==    by 0x77893B: symbol_file_add_from_bfd (symfile.c:1280)
==7261==    by 0x8E51E3: solib_read_symbols (solib.c:706)
==7261==    by 0x8E58AF: solib_add (solib.c:1029)

This occurs because commit 1be5090b in bfd, addressing PR 11983, started
taking a copy of the input filename instead of directly caching it.  It
appears that this code was never updated to reflect that API change.

This simple patch creates a cleanup to free the return value for lrealpath.

gdb/ChangeLog

	* build-id.c (build_id_to_debug_bfd): Add cleanup to free
	return value from lrealpath.