modemmanager: fix compilation

modemmanager: fix compilation

MM has a problem with cross-compiling. It needs to run some
tools during compilation (glib-mkenums & gdbus-codegen) but uses
pkg-config to detect them. But like this it finds the wrong tools, the
ones in $(STAGING_DIR). The correct tools are in
$(STAGING_DIR_HOSTPKG)/bin.

As a workaround this commit patches configure.ac so the correct tools
are used, the ones from glib2/host. The latter is also added to
PKG_BUILD_DEPENDS to reflect this.

libxslt/host is also added to PKG_BUILD_DEPENDS and the related
RequireCommand call is removed. This is OK to do since we have
libxslt/host available since a few days ago.

Resolves #10249

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>