string.h: adjust memchr return value

C defines the return value without const;
see https://en.cppreference.com/w/c/string/byte/memchr.

Change-Id: I65faf54d748f2eb2cf6bc9296686006a572592ce

sdk/include/string.h

diff --git a/sdk/include/string.h b/sdk/include/string.h
index d5dac35..5261654 100644
--- a/sdk/include/string.h
+++ b/sdk/include/string.h
@@ -10,7 +10,7 @@
 void *__cheri_libcall memcpy(void *dest, const void *src, size_t n);
 void *__cheri_libcall memset(void *, int, size_t);
 void *__cheri_libcall memmove(void *dest, const void *src, size_t n);
-const void *__cheri_libcall memchr(const void *, int, size_t);
+void *__cheri_libcall memchr(const void *, int, size_t);
 size_t __cheri_libcall      strlen(const char *str);
 int __cheri_libcall         strncmp(const char *s1, const char *s2, size_t n);
 char *__cheri_libcall       strncpy(char *dest, const char *src, size_t n);

sdk/lib/string/memchr.c

diff --git a/sdk/lib/string/memchr.c b/sdk/lib/string/memchr.c
index eb08f5e..7961976 100644
--- a/sdk/lib/string/memchr.c
+++ b/sdk/lib/string/memchr.c
@@ -3,9 +3,9 @@
 
 #include <string.h>
 
-const void *__cheri_libcall memchr(const void *voidString,
-                                   int         intChar,
-                                   size_t      n)
+void *__cheri_libcall memchr(const void *voidString,
+                             int         intChar,
+                             size_t      n)
 {
 	const unsigned char  c = (unsigned char)intChar;
 	const unsigned char *s = (const unsigned char *)voidString;
@@ -14,7 +14,7 @@
 	{
 		if (*s == c)
 		{
-			return (const void *)s;
+			return (void *)s;
 		}
 		s++;
 	}