meson: bpf: propagate 'sysroot' for cross compilation

During cross-compilation of systemd, the compiler used to build the bpf's needs
to be pointed at the correct include searchpath. Which can be done by passing
the corresponding directory in through the cflags; for example in yocto/bitbake
this would work: CFLAGS += "--sysroot=${STAGING_DIR_TARGET}"

Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>

meson.build

@@ -1740,6 +1740,7 @@ if conf.get('BPF_FRAMEWORK') == 1
                 '-ffile-prefix-map=',
                 '-fdebug-prefix-map=',
                 '-fmacro-prefix-map=',
+                '--sysroot=',
         ]
 
         foreach opt : c_args