render/color: add fallback stub when LCMS2 is disabled

It's cumbersome for compositors to guard every
wlr_color_transform_ref() or wlr_color_transform_unref() call
behind a #if WLR_HAS_COLOR_MANAGEMENT. Moreover, none of the LCMS2
types are used in our public API.

Instead, always install the color.h header, and add a stub for
wlr_color_transform_init_linear_to_icc().

render/color_fallback.c

@@ -0,0 +1,9 @@
+#include <wlr/render/color.h>
+#include <wlr/util/log.h>
+
+struct wlr_color_transform *wlr_color_transform_init_linear_to_icc(
+		const void *data, size_t size) {
+	wlr_log(WLR_ERROR, "Cannot create color transform from ICC profile: "
+		"LCMS2 is compile-time disabled");
+	return NULL;
+}