Add missing dependency to src/meson.build so KMSCON builds successfully

[sidstuff]

Hello, I'd like to submit a small fix as currently it seems the repo does not build successfully (tested on Ubuntu Server 25.04 with all required packages installed).

I ran meson buillddir/ followed by sudo meson install -C builddir/, the latter of which failed, printing:

[42/64] Compiling C object src/kmscon.p/kmscon_terminal.c.o
FAILED: src/kmscon.p/kmscon_terminal.c.o 
cc -Isrc/kmscon.p -Isrc -I../src -Iexternal -I../external -I/usr/local/include -I/usr/include/libdrm -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Werror -std=gnu99 -O2 -g -ffast-math -fno-strict-aliasing -ffunction-sections -fdata-sections -fstack-protector -fvisibility=hidden -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L -include /home/sid/kmscon/builddir/config.h -pthread -MD -MQ src/kmscon.p/kmscon_terminal.c.o -MF src/kmscon.p/kmscon_terminal.c.o.d -o src/kmscon.p/kmscon_terminal.c.o -c ../src/kmscon_terminal.c
../src/kmscon_terminal.c:62:10: fatal error: dbus/dbus.h: No such file or directory
   62 | #include <dbus/dbus.h>
      |          ^~~~~~~~~~~~~
compilation terminated.

The error shows that the required -I flags for dbus-1 (given by pkg-config --cflags dbus-1) were not included in the cc command—but those for libdrm were.

So searching the build files for where libdrm was included as a dependency but dbus-1 wasn't, it seems to be uterm_dep in src/meson.build.

And indeed, adding dbus_deps to uterm_dep makes KMSCON build successfully; see the new output of sudo meson install -C builddir/.

Fixes #1