cgraph storeFileName: squash warnings when dealing with buffer lengths

cgraph storeFileName: squash warnings when dealing with buffer lengths

This removes the following compiler warnings which were particularly problematic
as they were pointing to the wrong source lines. It is unclear whether this is
due to a Flex bug or the way we are calling Flex.

  ../../lib/cgraph/scan.l: In function ‘storeFileName’:
  ../../lib/cgraph/scan.l:98:28: warning: conversion to ‘size_t’ {aka ‘long
    unsigned int’} from ‘int’ may change the sign of the result
    [-Wsign-conversion]
     98 |     static char* buf;
        |                            ^
  ../../lib/cgraph/scan.l:98:37: warning: conversion to ‘size_t’ {aka ‘long
    unsigned int’} from ‘int’ may change the sign of the result
    [-Wsign-conversion]
     98 |     static char* buf;
        |                                     ^
  ../../lib/cgraph/scan.l: In function ‘ppDirective’:
  ../../lib/cgraph/scan.l:125:29: warning: conversion from ‘long int’ to ‘int’
    may change value [-Wconversion]
    125 |             while (*e && *e != '"') e++;
        |                            ~^~