string: Fix wrong comparison semantics

Chars are signed for x86 -- correct the comparison semantics.

Change-Id: I2049e98eb063c0b4e83ea973d3fcae49c6817dde
Author: Liubov Dmitrieva <liubov.dmitrieva@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>

libc/string/strchr.c

@@ -34,7 +34,7 @@ char *
 strchr(const char *p, int ch)
 {
 	for (;; ++p) {
-		if (*p == ch)
+		if (*p == (char) ch)
 			return((char *)p);
 		if (!*p)
 			return((char *)NULL);