yabar: fix build

The stable build for `yabar` is currently broken: https://hydra.nixos.org/build/75989172

Main reason is that the inline function `ya_setup_prefix_suffix` is
supposed to be an inline function, but was insufficiently declared as
such which broke the compiler recently with the following message:

```
gcc -std=c99 -Iinclude -pedantic -Wall -Os `pkg-config --cflags pango pangocairo libconfig` -DVERSION=\"0.4.0\" -D_POSIX_C_SOURCE=199309L -DYA_INTERNAL -DYA_DYN_COL -DYA_ENV_VARS -DYA_INTERNAL_EWMH  -c -o src/intern_blks/ya_intern.o src/intern_blks/ya_intern.c
gcc -o yabar src/ya_main.o src/ya_draw.o src/ya_exec.o src/ya_parse.o src/intern_blks/ya_intern.o -lxcb -lpthread -lxcb-randr -lxcb-ewmh `pkg-config --libs pango pangocairo libconfig`
src/intern_blks/ya_intern.o: In function `ya_int_date':
ya_intern.c:(.text+0x49): undefined reference to `ya_setup_prefix_suffix'
src/intern_blks/ya_intern.o: In function `ya_int_uptime':
ya_intern.c:(.text+0xf4): undefined reference to `ya_setup_prefix_suffix'
src/intern_blks/ya_intern.o: In function `ya_int_brightness':
ya_intern.c:(.text+0x1d8): undefined reference to `ya_setup_prefix_suffix'
src/intern_blks/ya_intern.o: In function `ya_int_bandwidth':
ya_intern.c:(.text+0x377): undefined reference to `ya_setup_prefix_suffix'
src/intern_blks/ya_intern.o: In function `ya_int_cpu':
ya_intern.c:(.text+0x6de): undefined reference to `ya_setup_prefix_suffix'
src/intern_blks/ya_intern.o:ya_intern.c:(.text+0x924): more undefined references to `ya_setup_prefix_suffix' follow
collect2: error: ld returned 1 exit status
make: *** [Makefile:18: yabar] Error 1
```

This issue has been fixed on master (see
https://github.com/geommer/yabar/commit/9779a5e04bd6e8cdc1c9fcf5d7ac31416af85a53)
which is why `nixos.yabar-unstable` remained functional.