meson: Fix yajl workaround

meson: Fix yajl workaround

Meson reports

  WARNING: pkgconfig variable 'cflags' not defined for dependency yajl.

which makes sense, because "cflags" is not one of the variables
reported by

  $ pkg-config --print-variables yajl

and

  $ pkg-config --variable=cflags yajl

doesn't work either.

The breakage was introduced when we switched from calling
pkg-config directly to using get_pkgconfig_variable() in 7.5.0
and, somehow, it went undetected until now.

Use "includedir", which is a proper pkg-config variable,
instead.

Fixes: c32c5ca29aeb63d329e2c1e60e249246c010f2f3
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>