Make egl_error function part of the egl interface

The function may be useful for backends using egl.

backend/egl.c

@@ -6,7 +6,7 @@
 #include "backend/egl.h"
 #include "common/log.h"
 
-static const char *egl_error(void) {
+const char *egl_error(void) {
 	switch (eglGetError()) {
 	case EGL_SUCCESS:
 		return "Success";

include/backend/egl.h

@@ -10,6 +10,7 @@ struct wlr_egl {
 	EGLContext context;
 };
 
+const char *egl_error(void);
 bool wlr_egl_init(struct wlr_egl *egl, EGLenum platform, void *display);
 void wlr_egl_free(struct wlr_egl *egl);
 EGLSurface wlr_egl_create_surface(struct wlr_egl *egl, void *window);