PGI complains about Derf symbol in module_mp_SBM_polar_radar.F

[nbald]

Hi

I got this error when compiling WRF 4.2 with PGI 19.10:

441044-PGF90-S-0038-Symbol, derf, has not been explicitly declared (module_mp_SBM_polar_radar.f90)
441136-  0 inform,   0 warnings,   1 severes, 0 fatal for calc_dc_wet_snow

I added REAL::DERF after "implicit none" in module_mp_SBM_polar_radar.F, on line 1506 https://github.com/wrf-model/WRF/blob/0d88b5800186d892a882e118425d2e684ded756f/phys/module_mp_SBM_polar_radar.F#L1505

After that, the WRF build was successful

[davegill]

@nbald take a look at #1246