Merge pull request #2012 from yshui/musl_libc_src_core_sys_posix_stdio

 Add musl libc definitions to:  src_core_sys_posix_stdio
merged-on-behalf-of: Iain Buclaw <ibuclaw@gdcproject.org>

Author: dlang-bot

src/core/sys/posix/stdio.d

@@ -285,6 +285,10 @@ version( CRuntime_Glibc )
 {
     enum P_tmpdir  = "/tmp";
 }
+version( CRuntime_Musl )
+{
+    enum P_tmpdir  = "/tmp";
+}
 version( Darwin )
 {
     enum P_tmpdir  = "/var/tmp";