Add dlext.h to the libdl doc group.

Test: https://irina-dot-devsite.googleplex.com/ndk/reference/group/libdl
Bug: http://b/77236573
Change-Id: Ie09351ee961554488e34c79511712b07c345b8ad
diff --git a/libc/include/android/dlext.h b/libc/include/android/dlext.h
index 2b4169e..d0200c4 100644
--- a/libc/include/android/dlext.h
+++ b/libc/include/android/dlext.h
@@ -24,6 +24,11 @@
 #include <sys/types.h>  /* for off64_t */
 
 /**
+ * @addtogroup libdl Dynamic Linker
+ * @{
+ */
+
+/**
  * \file
  * Advanced dynamic library opening support. Most users will want to use
  * the standard [dlopen(3)](http://man7.org/linux/man-pages/man3/dlopen.3.html)
@@ -179,4 +184,6 @@
 
 __END_DECLS
 
+/** @} */
+
 #endif