Include sys/types.h in c_src/erl_blf.h

This is needed to compile on Alpine Linux.

Thanks anthonyrisinger.
diff --git a/c_src/erl_blf.h b/c_src/erl_blf.h
index d64535f..b03a3a0 100644
--- a/c_src/erl_blf.h
+++ b/c_src/erl_blf.h
@@ -42,6 +42,8 @@
 #define u_int64_t uint64_t
 #endif
 
+#include <sys/types.h>
+
 /* Schneier specifies a maximum key length of 56 bytes.
  * This ensures that every key bit affects every cipher
  * bit.  However, the subkeys can hold up to 72 bytes.