mux_isom: accept integer 48 fps for ProRes muxing

Follow-up to #3544. That fix added 48000/1001 (NTSC 47.952) to the
filter-side validator in src/filters/mux_isom.c, and added BOTH 47.952
and integer 48 to the post-mux validator gf_media_check_qt_prores() in
src/media_tools/isom_tools.c (ifps ranges 4794..4796 and 4799..4801).

The mux_isom.c gate fires first during filter graph connection when the
input is a playlist, so integer-48 ProRes PIDs are rejected with
"[ProRes] Unrecognized frame rate 48" before the post-mux validator is
ever reached. The isom_tools.c integer-48 entry is therefore
unreachable on the filter path today. Add the missing line to keep the
two validators symmetric.