meson: patch libass with stub dirent.h

this should probably be handled by the libass meson port at some point

subprojects/libass.wrap

@@ -2,3 +2,4 @@
 directory = libass
 url = https://github.com/TypesettingTools/libass.git
 revision = meson-no-rasterizer-approximation
+patch_directory = libass

subprojects/packagefiles/libass/dirent.h

@@ -0,0 +1,6 @@
+// libass uses dirent in a function we don't use, so just provide a dummy version
+typedef struct DIR { int dummy; } DIR;
+typedef struct dirent { char *d_name; } dirent;
+static inline DIR *opendir(const char *x) { return 0; }
+static inline struct dirent *readdir(DIR *x) { return 0; }
+static inline void closedir(DIR *x) { }