gdb.base/{exprs,ptype,ptype1,setvar,whatis}.c C++ify

gdb.base/{exprs,ptype,ptype1,setvar,whatis}.c C++ify

This adjusts:

 gdb.base/{exprs,ptype,ptype1,setvar,whatis}.c,

to make them buildable as C++ programs.

gdb/testsuite/ChangeLog:

	* gdb.base/exprs.c: Replace 'this' with 'self' throughout.
	* gdb.base/ptype.c: : Replace 'this' with 'self' throughout.
	(charfoo, intfoo): Define full prototype.
	* gdb.base/ptype1.c (charfoo): Define full prototype.
	* gdb.base/setvar.c: Replace 'this' with 'self' throughout.
	* gdb.base/whatis.c: Replace 'this' with 'self' throughout.