package/gobject-introspection: fix exec_prefix

package/gobject-introspection: fix exec_prefix

exec_prefix has been removed since version 1.66 and
https://gitlab.gnome.org/GNOME/gobject-introspection/-/commit/a88b1ac548516ab25140a2e9dea8cac894bbac93
resulting in the following build failure with rygel because libgee
installs Gee-0.8.gir in
output/host/aarch64-buildroot-linux-gnu/sysroot/share/gir-1.0/Gee-0.8.gir
instead of
output/host/aarch64-buildroot-linux-gnu/sysroot/usr/share/gir-1.0/Gee-0.8.gir:

Could not find GIR file 'Gee-0.8.gir'; check XDG_DATA_DIRS or use --includedir
error parsing file RygelCore-2.6.gir: Failed to parse included gir Gee-0.8
If the above error message is about missing .so libraries, then setting up GIR_EXTRA_LIBS_PATH in the .mk file should help.
Typically like this: PKG_MAKE_ENV += GIR_EXTRA_LIBS_PATH="$(@D)/.libs"

So replace exec_prefix by prefix

Fixes:
 - http://autobuild.buildroot.org/results/b04b27bbb84fa6ab9a8441e7bd94aad45226dce9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>