plugins: Force types for isalnum and isblank

plugins: Force types for isalnum and isblank

The two users of isalnum and isblank hand in unsigned char and not
int as expected by the API. This triggers thye type checker in the
debug build, e.g.

../subprojects/libnvme/ccan/ccan/build_assert/build_assert.h:38:22: error: size of unnamed array is negative
   38 |         (sizeof(char [1 - 2*!(cond)]) - 1)
      |                      ^
../ccan/ccan/str/str.h:189:16: note: in expansion of macro ‘BUILD_ASSERT_OR_ZERO’
  189 |         ((i) + BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(typeof(i), \
      |                ^~~~~~~~~~~~~~~~~~~~
../ccan/ccan/str/str.h:196:32: note: in expansion of macro ‘str_check_arg_’
  196 | #define isalnum(i) str_isalnum(str_check_arg_(i))
      |                                ^~~~~~~~~~~~~~
../plugins/memblaze/memblaze-nvme.c:410:13: note: in expansion of macro ‘isalnum’
  410 |         if (isalnum(*c) == 0) {
      |             ^~~~~~~

Signed-off-by: Daniel Wagner <dwagner@suse.de>