target.c: Remove struct keyword in range-based for

target.c: Remove struct keyword in range-based for

I get this when compiling with a gcc 6.3.0-based cross-compiler:

  CXX    target.o
/home/simark/src/binutils-gdb/gdb/target.c: In static member function 'static void target_terminal::restore_inferior()':
/home/simark/src/binutils-gdb/gdb/target.c:396:10: error: types may not be defined in a for-range-declaration [-Werror]
     for (struct inferior *inf : all_inferiors ())
          ^~~~~~

Accomodate it by dropping the unnecessary struct keyword.  Actually, I used
"::inferior", otherwise it resolves to the inferior method of the
target_terminal class.

gdb/ChangeLog:

	* target.c (target_terminal::restore_inferior): Remove struct keyword.