v1 header inclusion issue on Windows

[hermet]

@hermet

$ pkgconf --cflags thorvg-1
-IC:/Documents/msys2_2/opt/thorvg/include/thorvg-1 -IC:/Documents/msys2_2/home/vincent.torri/ewpi_64/include/libpng16 -IC:/Documents/msys2_2/home/vincent.torri/
ewpi_64/include -IC:/Documents/msys2_2/ucrt64/bin/../include -IC:/Documents/msys2_2/home/vincent.torri/ewpi_64/include/webp

including thorvg-1/thorvg.h is not possible, hence my change, to at least compile

Originally posted by @vtorri in #16 (comment)

[vtorri]

is there -IC:/Documents/msys2_2/opt/thorvg/include in --cflags on inux ?