use LIBUV_INC so that USE_SYSTEM_LIBUV works

`LIBUV_INC` gets set here https://github.com/JuliaLang/julia/blob/master/Make.inc#L404-L410

When `USE_SYSTEM_LIBUV` is not 1, we have `LIBUV_INC = $(JULIAHOME)/deps/libuv/include` so these are equivalent. But when it is 1, `$(JULIAHOME)/deps/libuv` may not exist at all and if the paths need manual modification, it's much nicer to `override LIBUV_INC` in Make.user than `override HEADERS`.