Add syncfs(2).

GMM calls this system call directly at the moment. That's silly.

Bug: http://b/36405699
Test: ran tests
Change-Id: I1e14c0e5ce0bc2aa888d884845ac30dc20f13cd5
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index d674630..a1d1af0 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -131,6 +131,7 @@
 int         fchown:fchown32(int, uid_t, gid_t)  arm,x86
 int         fchown:fchown(int, uid_t, gid_t)    arm64,mips,mips64,x86_64
 void        sync(void)  all
+int         syncfs(int)  all
 int         ___fsetxattr:fsetxattr(int, const char*, const void*, size_t, int) all
 ssize_t     ___fgetxattr:fgetxattr(int, const char*, void*, size_t) all
 ssize_t     ___flistxattr:flistxattr(int, char*, size_t) all