Fix meson pkg-config generation

[rsmyth-ecmi]

The pkg-config file does not include the subdir in its build flags, so files will fail to find the Unity headers. Basically the previous file looked like this:

prefix=/usr/lib/unity-test-2.6.1
includedir=${prefix}/include
libdir=${prefix}/lib

Name: unity
Description: C Unit testing framework.
Version: 2.6.1
Libs: -L${libdir} -lunity
Cflags: -I${includedir}

The new one looks like this

prefix=/usr/lib/unity-test-2.6.1
includedir=${prefix}/include
libdir=${prefix}/lib

Name: unity
Description: C Unit testing framework.
Version: 2.6.1
Libs: -L${libdir} -lunity
Cflags: -I${includedir}/unity

Notice the Cflags variable.

[rsmyth-ecmi]

@mvandervoord Would it be possible for this to be looked at? .pc file generation is currently busted without it.