gdb: use decltype instead of typeof in dwarf2/read.c

gdb: use decltype instead of typeof in dwarf2/read.c

When building with -std=c++11, I get:

  CXX    dwarf2/read.o
/home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c: In function ‘void dwarf2_build_psymtabs_hard(dwarf2_per_objfile*)’:
/home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:7130:23: error: expected type-specifier before ‘typeof’
 7130 |     using iter_type = typeof (per_bfd->all_comp_units.begin ());
      |                       ^~~~~~

This is because typeof is a GNU extension.  Use C++'s decltype keyword
instead.

Change-Id: Ieca2e8d25e50f71dc6c615a405a972a54de3ef14