Update to libvips 8.15.0

[znarf]

The latest version of libvips is now 8.15.0 and the latest version of sharp (0.33.0) requires it.

I personally attempted to do it running: VIPS_VERSION=8.15.0 ./build.sh

And got the following error:

#13 4.181 The Meson build system
#13 4.181 Version: 1.3.0
#13 4.181 Source dir: /usr/local/src/libvips-8.15.0
#13 4.181 Build dir: /usr/local/src/libvips-8.15.0/build
#13 4.181 Build type: native build
#13 4.181 
#13 4.181 meson.build:1:0: ERROR: Value "false" (of type "string") for combo option "Build GObject introspection data" is not one of the choices. Possible choices are (as string): "enabled", "disabled", "auto".