This avoids relying on the `includes` target to generate this header
which must be in .OBJDIR for the ncurses library to find it.
With an upcoming change libcompat build won't build a seperate
sysroot and will instead share with the 64-bit target which means the
`includes` target will not be called.
Sponsored by: DARPA, AFRL
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35510
b0bcedef1a