Fix gdb.base/structs2.exp with Clang

gdb.base/structs2.exp fails to run with Clang, because of:

 gdb compile failed, /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/structs2.c:16:14: warning:
       implicit conversion from 'int' to 'signed char' changes value from 130 to
       -126 [-Wconstant-conversion]
   param_reg (130, 120, 33000, 32000);
   ~~~~~~~~~  ^~~
 /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/structs2.c:16:24: warning:
       implicit conversion from 'int' to 'short' changes value from 33000 to
       -32536 [-Wconstant-conversion]
   param_reg (130, 120, 33000, 32000);
   ~~~~~~~~~            ^~~~~
 2 warnings generated.

		 === gdb Summary ===

 # of untested testcases         1

Fix it by passing actual negative numbers.

gdb/testsuite/ChangeLog:

	* gdb.base/structs2.c (main): Adjust second parem_reg call to
	explicitly write negative numbers.
	* gdb.base/structs2.exp: Adjust expected output.