Move exit.c from stdlib to bionic subdirectory.

Test: make

Change-Id: I2182d5a7c97abc0335e88d2d9dd9f77bc7e7f633
diff --git a/libc/Android.bp b/libc/Android.bp
index 4e5ae75..c3afa3a 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -4,6 +4,7 @@
     "async_safe/async_safe_log.cpp",
     "bionic/ether_aton.c",
     "bionic/ether_ntoa.c",
+    "bionic/exit.cpp",
     "bionic/fts.c",
     "bionic/initgroups.c",
     "bionic/isatty.c",
@@ -17,7 +18,6 @@
     "stdio/stdio_ext.cpp",
     "stdio/vfscanf.cpp",
     "stdio/vfwscanf.c",
-    "stdlib/exit.c",
 ]
 
 // off64_t/time64_t support on LP32.