Don't include symtab.h from expression.h

expression.h includes symtab.h, but apparently only for the
declaration of struct block.  This patch changes it to foward-declare
the structure, and remove the include.

gdb/ChangeLog
2019-03-24  Tom Tromey  <tom@tromey.com>

	* expression.h: Don't include symtab.h.
	(struct block): Forward declare.

Author: tromey

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-03-24  Tom Tromey  <tom@tromey.com>
+
+	* expression.h: Don't include symtab.h.
+	(struct block): Forward declare.
+
 2019-03-24  Tom Tromey  <tom@tromey.com>
 
 	* c-exp.y (typebase): Remove casts.

gdb/expression.h

@@ -20,9 +20,6 @@
 #if !defined (EXPRESSION_H)
 #define EXPRESSION_H 1
 
-
-#include "symtab.h"		/* Needed for "struct block" type.  */
-
 /* While parsing expressions we need to track the innermost lexical block
    that we encounter.  In some situations we need to track the innermost
    block just for symbols, and in other situations we want to track the