Isolation: fixed the generation of mounts table.

Since the introduction of rootfs feature, some language modules
can't be configured multiple times.

Now the configure generates a separate nxt_<module>_mounts.h for
each module compiled.

src/ruby/nxt_ruby.c

@@ -7,7 +7,8 @@
 
 #include <nxt_unit.h>
 #include <nxt_unit_request.h>
-#include <nxt_ruby_mounts.h>
+
+#include NXT_RUBY_MOUNTS_H
 
 
 #define NXT_RUBY_RACK_API_VERSION_MAJOR  1