Add check for #include of header file without path

[meheff]

From #1972

#include of file without directory:

#include "c_api_vast.h"

[hzeller]

In the CI, We need a script like

#!/bin/sh
find xls -name "*.h" -o -name "*.cc"  \
   | xargs egrep -n '#include "[^/]*"' \
   | egrep -v "synth_only|linenoise|%s|xls_fixed|xls_int"
if [ $? -eq 0 ]; then
  echo "::error:: always use a fully qualified name for #includes"
  exit 1 
fi

It would then generate an output like this

xls/public/c_api.cc:51:#include "c_api_vast.h"
::error:: always use a fully qualified name for #includes

The ::error:: makes it look highlighted in the CI log.