make is_scalar_type non-static and use it in ada-lang.c

make is_scalar_type non-static and use it in ada-lang.c

Just a small cleanup, to avoid code duplication...

gdb/ChangeLog:

        * gdbtypes.h (is_scalar_type): Add extern declaration.
        * gdbtypes.c (is_scalar_type): Make non-static.
        * ada-lang.c (ada_value_primitive_packed_val): Use is_scalar_type
        to compute IS_SCALAR instead of doing it ourselves.