libc: create poll.h symlink

Since user-land tools tend to look for <poll.h> instead of <sys/poll.h>
make them happy like w/o resorting to patch the source.

Fixes #1356.

repos/libports/ports/libc.hash

@@ -1 +1 @@
-4c562d752cdcb25acd42ad285a4c544d4863a22c
+1f3035bb342960e15effef162cf4d2f50fbb7d7f

repos/libports/ports/libc.port

@@ -234,3 +234,13 @@ generated_files += \
 # generate files before assembling the public include directory tree
 _dirs: $(generated_files)
 $(generated_files) : $(DOWNLOADS)
+
+
+#
+# Create symlinks for sys headers expected in parent directory
+#
+
+default: create_symlinks
+create_symlinks: _dirs
+	@$(MSG_PREFIX)create symlinks
+	$(VERBOSE)ln -sf sys/poll.h include/libc/poll.h