Api Symbols: hide furi_background in f18

Author: skotopes

targets/f18/api_symbols.csv

@@ -1102,7 +1102,7 @@ Function,-,ftello,off_t,FILE*
 Function,-,ftrylockfile,int,FILE*
 Function,-,funlockfile,void,FILE*
 Function,-,funopen,FILE*,"const void*, int (*)(void*, char*, int), int (*)(void*, const char*, int), fpos_t (*)(void*, fpos_t, int), int (*)(void*)"
-Function,+,furi_background,void,
+Function,-,furi_background,void,
 Function,+,furi_delay_ms,void,uint32_t
 Function,+,furi_delay_tick,void,uint32_t
 Function,+,furi_delay_until_tick,FuriStatus,uint32_t