meson: fix building bin/override.c when X exists

cc -Ibin/vlc.p -Ibin -I../bin -I. -I.. -Iinclude -I../include -I../compat/stdbit -I../compat/stdckdint -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=gnu17 -O0 -g -DHAVE_CONFIG_H=1 -Wno-deprecated-copy -Wextra -Wsign-compare -Wundef -Wpointer-arith -Wvolatile-register-var -Wformat -Wformat-security -Wbad-function-cast -Wwrite-strings -Wmissing-prototypes -Werror-implicit-function-declaration -Winit-self -pipe -mbranch-protection=standard -Wall -Werror-implicit-function-declaration -fvisibility=hidden -pthread -MD -MQ bin/vlc.p/override.c.o -MF bin/vlc.p/override.c.o.d -o bin/vlc.p/override.c.o -c ../bin/override.c
../bin/override.c:282:11: fatal error: 'X11/Xlib.h' file not found
          ^~~~~~~~~~~~
1 error generated.
[5/1929] Compiling C object bin/vlc-static.p/override.c.o
FAILED: bin/vlc-static.p/override.c.o
cc -Ibin/vlc-static.p -Ibin -I../bin -I. -I.. -Iinclude -I../include -I../compat/stdbit -I../compat/stdckdint -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -std=gnu17 -O0 -g -DHAVE_CONFIG_H=1 -Wno-deprecated-copy -Wextra -Wsign-compare -Wundef -Wpointer-arith -Wvolatile-register-var -Wformat -Wformat-security -Wbad-function-cast -Wwrite-strings -Wmissing-prototypes -Werror-implicit-function-declaration -Winit-self -pipe -mbranch-protection=standard -Wall -Werror-implicit-function-declaration -fvisibility=hidden -pthread '-DTOP_BUILDDIR="/home/brad/tmp/vlc/build"' '-DTOP_SRCDIR="/home/brad/tmp/vlc"' -MD -MQ bin/vlc-static.p/override.c.o -MF bin/vlc-static.p/override.c.o.d -o bin/vlc-static.p/override.c.o -c ../bin/override.c
../bin/override.c:282:11: fatal error: 'X11/Xlib.h' file not found
          ^~~~~~~~~~~~
1 error generated.