Fix gdb.base/charset.exp with Clang

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

 gdb compile failed, /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/charset.c:144:20: warning:
       implicit conversion from 'int' to 'char' changes value from 162 to -94
       [-Wconstant-conversion]
		11, 162, 17);
		    ^~~
 /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/charset.c:151:16: warning:
       implicit conversion from 'int' to 'char' changes value from 167 to -89
       [-Wconstant-conversion]
		167,
		^~~
 /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.base/charset.c:168:16: warning:
       implicit conversion from 'int' to 'char' changes value from 167 to -89
       [-Wconstant-conversion]
		167,
		^~~
 3 warnings generated.

		 === gdb Summary ===

 # of untested testcases         1

Fix it by changing init_string to take unsigned char parameters.

gdb/testsuite/ChangeLog:

	* gdb.base/charset.c (init_string): Change all char parameters to
	unsigned char parameters.