build,meson: fix pkg-config variable get function
When there is no udevdir
variable set when mbim_username
option is set, the variable is get from the udev.pc
pkg-config file.
However, the function used, this is get_variable
was introduced in meson 0.51. It has been replaced by get_pkgconfig_variable
that was already supported in meson 0.45.1.