It is /usr/local/include, yes. There is no include folder inside the Cellar folder.
ls /usr/local/include returns this: SDL2 evhttp.h gdbm.h hidparser.h libelf lua5.4 oniguruma.h stm32.h unigbrk.h unistring autosprintf.h evrpc.h gettext-po.h hidtypes.h libintl.h lzma osxfuse textstyle unilbrk.h unitypes.h dbm.h evutil.h gmp.h idn2.h libltdl lzma.h pcre2.h textstyle.h uniname.h uniwbrk.h evdns.h ftdi.h gmpxx.h jq.h libusb-1.0 mpf2mpfr.h pcre2posix.h unicase.h uninorm.h uniwidth.h event.h gawkapi.h hid.h jv.h ltdl.h mpfr.h stlink uniconv.h unistdio.h usb.h event2 gdbm-ndbm.h hidapi libavrdude.h lua oniggnu.h stlink.h unictype.h unistr.h utf8proc.h Hans
