cpprules.in: squash whitespace in generated files

CPP is used to generate files, but as cpp reads files from the build host the output has a number of blank lines at the beginning which varies depending on what GCC and friends is used.

Pathalogical example:

 $ cpp -undef -traditional /dev/null
 # 1 "/dev/null"
 # 1 "<built-in>"
 # 1 "<command-line>"
 # 31 "<command-line>"
 # 1 "/usr/include/stdc-predef.h" 1 3 4

 # 17 "/usr/include/stdc-predef.h" 3 4

 [ 40 blank line ]

 # 32 "<command-line>" 2
 # 1 "/dev/null"

So depending on the content of stdc-predef.h and what other headers CPP will load, the amount of whitespace in the generates files varies. This can result in differences in reproducible environments, and file conflicts in multilib environments.

As whitespace is irrelevant to these machine-readable files, extend the sed to just delete blank lines.