Fix build on solaris

[jwntree]

To resolve this:

sha1.c:210:40: error: implicit declaration of function 'strlen' [-Wimplicit-function-declaration]
  210 |     sha1_update(&ctx, (const BYTE*) s, strlen(s));
      |                                        ^~~~~~
sha1.c:197:1: note: include '<string.h>' or provide a declaration of 'strlen'
  196 | #include <stdint.h>
  +++ |+#include <string.h>
  197 |
sha1.c:210:40: warning: incompatible implicit declaration of built-in functio