Expose wait4 as wait4 rather than __wait4.

This helps strace(1) compile with one fewer hack.

Change-Id: I5296d0cfec5546709cda990abd705ad33d7c4626
diff --git a/libc/Android.mk b/libc/Android.mk
index b4613ba..9064907 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -203,7 +203,6 @@
 	bionic/unlockpt.c \
 	bionic/usleep.c \
 	bionic/utmp.c \
-	bionic/wait.c \
 	bionic/wcscoll.c \
 	netbsd/gethnamaddr.c \
 	netbsd/inet/nsap_addr.c \
@@ -285,6 +284,7 @@
     bionic/__umask_chk.cpp \
     bionic/__vsnprintf_chk.cpp \
     bionic/__vsprintf_chk.cpp \
+    bionic/wait.cpp \
     bionic/wchar.cpp \
 
 libc_upstream_freebsd_src_files := \