Remove a warning from discarded-qualifiers

I've been seeing this for a while now:
```
../../../../ext/prism/../../src/memchr.c: In function ‘pm_memchr’:
../../../../ext/prism/../../src/memchr.c:35:16: warning: return discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   35 |         return memchr(memory, character, number);
      |                ^~~~~~
```

Author: Earlopain

include/prism/internal/memchr.h

@@ -10,6 +10,6 @@
  * we need to search for a character in a buffer that could be the trailing byte
  * of a multibyte character.
  */
-void * pm_memchr(const void *source, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding);
+const void * pm_memchr(const void *source, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding);
 
 #endif

src/memchr.c

@@ -11,7 +11,7 @@
  * we need to search for a character in a buffer that could be the trailing byte
  * of a multibyte character.
  */
-void *
+const void *
 pm_memchr(const void *memory, int character, size_t number, bool encoding_changed, const pm_encoding_t *encoding) {
     if (encoding_changed && encoding->multibyte && character >= TRAILING_BYTE_MINIMUM) {
         const uint8_t *source = (const uint8_t *) memory;