Fix path for errno.h

Fix warnings when built with musl libc by using the correct path for
errno.h as defined in POSIX.

Fixes the following warning:
In file included from /home/ncopa/src/exiv2/src/http.cpp:74:
/usr/include/sys/errno.h:1:2: warning: #warning redirecting incorrect #include <sys/errno.h> to <errno.h> [-Wcpp]
 #warning redirecting incorrect #include <sys/errno.h> to <errno.h>
  ^~~~~~~

src/http.cpp

@@ -71,7 +71,7 @@
 
 #include <unistd.h>
 #include <strings.h>
-#include <sys/errno.h>
+#include <errno.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <sys/uio.h>