pio: Add 'pragma once' to generated header files (#237)

We can't really use traditional include guards, because the header file may be piped to stdout (which means we might not have a file name).

tools/pioasm/c_sdk_output.cpp

@@ -68,6 +68,8 @@ struct c_sdk_output : public output_format {
 
         header(out, "This file is autogenerated by pioasm; do not edit!");
 
+        fprintf(out, "#pragma once\n");
+        fprintf(out, "\n");
         fprintf(out, "#if !PICO_NO_HARDWARE\n");
         fprintf(out, "#include \"hardware/pio.h\"\n");
         fprintf(out, "#endif\n");