Fix dependency on Js_obj in Jsx module

[cknitt]

While working on another feature, I got a build error on Windows CI:

rescript: [86/988] others/jsx.cmi
FAILED: others/jsx.cmi
D:\a\rescript-compiler\rescript-compiler\win32\bsc.exe -bs-cmi -bs-cmj -no-keep-locs -no-alias-deps -bs-no-version-header -bs-no-check-div-by-zero -nostdlib  -bs-cross-module-opt -make-runtime   -nopervasives  -unsafe  -w +50 -warn-error A -open Belt_internals   -I others  others/jsx.ml
File "others/jsx.ml", line 42, characters 14-27:
Error: The module Js.Obj is an alias for module Js_obj, which is missing

The problem is that jsx.ml uses Js.Obj.assign, but the dependency on the Js_obj module is not added to the ninja file.

As a workaround, I changed Js.Obj.assign to Js_obj.assign which caused the dependency to be picked up correctly.