Remove more "struct" keywords in range-based for loops

Remove more "struct" keywords in range-based for loops

GCC 6.3.0 produces this kind of errors:

  CXX    dwarf2read.o
/home/simark/src/binutils-gdb/gdb/dwarf2read.c: In function 'void process_cu_includes(dwarf2_per_objfile*)':
/home/simark/src/binutils-gdb/gdb/dwarf2read.c:10220:8: error: types may not be defined in a for-range-declaration [-Werror]
   for (struct dwarf2_per_cu_data *iter : dwarf2_per_objfile->just_read_cus)
        ^~~~~~

Removing the struct keyword makes it happy.

gdb/ChangeLog:

	* dwarf2read.c (process_cu_includes): Remove struct keyword.
	* serial.c (serial_interface_lookup): Remove struct keyword.