cmUVHandlePtr: Fix conversion to bool on Oracle Studio compiler

The operator added by commit 17690558c3 (cmUVHandlePtr: Add explicit
conversion to bool, 2023-10-26) works in direct expressions like
`if(foo)` but not compound expressions like `if(foo && ...)`.
Drop the `explicit` mark when compiling with Oracle Studio so we
can at least compile valid code.