printf-frexp.c evokes shadowing warning

[Jim Meyering]

Hi Bruno,

printf-frexp.c evokes a shadowing warning on at least a debian unstable
system, and now that coreutils uses it (via vasprintf-posix), it causes
the "make distcheck" build to fail:

  printf-frexp.c: In function 'printf_frexp':
  printf-frexp.c:65: error: declaration of 'exp' shadows a global declaration
  /usr/include/bits/mathcalls.h:101: error: shadowed declaration is here
  make[5]: *** [printf-frexp.o] Error 1

Please rename the parameter, e.g.,
("exponent" is already taken)

  sed -i 's/\<exp\>/exp_param/' printf-frexp.c