Rename the header <janet/janet.h> to <janet.h>

Makes it easier to use and remember, and makes the
variable `module/*headerpath*` make more sense.

src/webclient/main.c

@@ -20,7 +20,7 @@
 * IN THE SOFTWARE.
 */
 
-#include <janet/janet.h>
+#include <janet.h>
 #include <emscripten.h>
 
 extern const unsigned char *janet_gen_webinit;