dwarf2read: fix compilation issue with gcc 4.8

dwarf2read: fix compilation issue with gcc 4.8

gcc 4.8 (and probably other versions too) doesn't like that the local
variable symbol_linkage has the same name as the enum class defined in
the same context:

      CXX    dwarf2read.o
    /home/smarchi/src/binutils-gdb/gdb/dwarf2read.c: In member function ‘dwarf2_per_cu_data* dw2_debug_names_iterator::next()’:
    /home/smarchi/src/binutils-gdb/gdb/dwarf2read.c:5850:22: error: ‘symbol_linkage’ is not a class, namespace, or enumeration
       } symbol_linkage = symbol_linkage::unknown;
                      ^

Rename the local variable to avoid this.

This problem was originally reported with the Netbsd builder on the
buildbot, which uses gcc 5.5, I believe.  I am not able to test it on
that builder right now, but chances are that the fix will work there
too.

gdb/ChangeLog:

	* dwarf2read.c (dw2_debug_names_iterator::next): Rename local
	variable symbol_linkage to symbol_linkage_.