gdb: use gdb:hash_enum as hash function in offset_map_type

gdb: use gdb:hash_enum as hash function in offset_map_type

When building with g++ 4.8, we get this error (just an excerpt, because
g++ outputs a very long error message):

      CXX    dwarf2/read.o
    ...
    /home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:14616:31:   required from here
    /usr/include/c++/4.8/bits/hashtable_policy.h:1070:12: error: invalid use of incomplete type ‘struct std::hash<sect_offset>’
         struct _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2,

This is the same problem and fix as in commit f23f598e28ad ("[gdb] Fix
build breaker with gcc 4.8").  Pass an explicit hash function rather
than relying on the default std::hash<sect_offset>.

gdb/ChangeLog:

	PR gdb/25881
	* dwarf2/read.c (offset_map_type): Use
	gdb:hash_enum<sect_offset> as hash function.