package/exiv2: drop unrecognized variable

EXIV2_ENABLE_LIBXMP has been dropped since version 0.27 and
https://github.com/Exiv2/exiv2/commit/2784b1f7f7ddcc66211e6cf492de1588aa6093d9

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

package/exiv2/exiv2.mk

@@ -30,10 +30,10 @@ EXIV2_CONF_OPTS += -DEXIV2_ENABLE_PNG=OFF
 endif
 
 ifeq ($(BR2_PACKAGE_EXIV2_XMP),y)
-EXIV2_CONF_OPTS += -DEXIV2_ENABLE_XMP=ON -DEXIV2_ENABLE_LIBXMP=ON
+EXIV2_CONF_OPTS += -DEXIV2_ENABLE_XMP=ON
 EXIV2_DEPENDENCIES += expat
 else
-EXIV2_CONF_OPTS += -DEXIV2_ENABLE_XMP=OFF -DEXIV2_ENABLE_LIBXMP=OFF
+EXIV2_CONF_OPTS += -DEXIV2_ENABLE_XMP=OFF
 endif
 
 EXIV2_DEPENDENCIES += $(TARGET_NLS_DEPENDENCIES)