gdb: Remove duplicate declaration of global innermost_block
    
    The global 'innermost_block' is declared in two header files.  Remove
    one of the declarations, and add an include of the other header into
    the one source file that could no longer see a declaration of
    'innermost_block'.
    
    gdb/ChangeLog:
    
    	* expression.h (innermost_block): Remove declaration.
    	* varobj.c: Add 'parser-defs.h' include.