fix disabling fortran

[tjhei]

I have fortran, but I am disabling it using WB_MAKE_FORTRAN_WRAPPER=OFF.

Without this patch I am getting

CMake Error: Error required internal CMake variable not set, cmake may not be built correctly.
Missing variable is:
CMAKE_Fortran_COMPILE_OBJECT

also see https://github.com/geodynamics/aspect/pull/5792/files

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.015 ± 0.004 (s=463)	1.020 ± 0.004 (s=424)	+0.4% .. +0.6%
Slab interpolation curved simple none	1.020 ± 0.005 (s=454)	1.025 ± 0.004 (s=429)	+0.4% .. +0.6%
Spherical slab interpolation simple none	1.185 ± 0.006 (s=370)	1.187 ± 0.006 (s=391)	+0.0% .. +0.3%
Slab interpolation simple curved CMS	1.059 ± 0.005 (s=407)	1.063 ± 0.006 (s=443)	+0.3% .. +0.5%
Spherical slab interpolation simple CMS	1.591 ± 0.010 (s=263)	1.593 ± 0.013 (s=305)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.187 ± 0.006 (s=388)	1.187 ± 0.006 (s=373)	-0.2% .. +0.1%
Cartesian min max surface	2.306 ± 0.018 (s=204)	2.309 ± 0.018 (s=189)	-0.2% .. +0.4%
Spherical min max surface	7.219 ± 0.032 (s=74)	7.224 ± 0.027 (s=53)	-0.2% .. +0.3%

[MFraters]

Looks good to me, thanks!