Move interpreter_p declaration to main.h

Move interpreter_p declaration to main.h

This moves the interpreter_p declaration from defs.h to main.h.  I
think this makes more sense, as it is defined in main.c.  Also, this
declaration was in the wrong place -- between a comment and the things
the comment described.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tromey@adacore.com>

	* tui/tui-interp.c: Include main.h.
	* interps.c: Include main.h.
	* main.h (interpreter_p): Declare.
	* defs.h (interpreter_p): Don't declare.