build: Define OSTREE_ENABLE_EXPERIMENTAL_API for g-ir-scanner

build: Define OSTREE_ENABLE_EXPERIMENTAL_API for g-ir-scanner

When compiling libostree, OSTREE_ENABLE_EXPERIMENTAL_API is managed via
config.h. However, g-ir-scanner can't use that since it gets confused
about the namespace of all the random macros. It won't include the
experimental APIs unless the macro is defined through another means.
Without this, none of the experimental APIs were being included in the
gir data.

Closes: #1322
Approved by: pwithnall