pmc: Use isascii(3) to check if a character is ASCIIHEADmain

The previous check did not work on architectures where `char` is
unsigned as noted by GCC on aarch64:

usr.sbin/pmc/view.cc: In member function 'void pmcview::loadsymboltable(image*, Elf*, Elf_Scn*, GElf_Shdr*)':
usr.sbin/pmc/view.cc:627:38: error: comparison is always false due to limited range of data type [-Werror=type-limits]
  627 |                         if (fname[i] < 0) {
      |                             ~~~~~~~~~^~~

Reported by:	GCC 15

0 files changed, 0 insertions, 0 deletions