Fix compile warnings (cygwin gcc)

[xtrnc]

Compiling with cygwin (gcc 10.2.0) on win10 fails because of "array
subscript has type 'char'" warnings. If isspace is called with a
negative char value it would invoke undefined behavior.

gcc -O3 -Wall -Werror -fpic -std=gnu99 -shared src/fzf.c -o build/libfzf.dll
In file included from src/fzf.c:4:
src/fzf.c: In function 'leading_whitespaces':
src/fzf.c:83:27: error: array subscript has type 'char' [-Werror=char-subscripts]
   83 |     if (!isspace(str->data[i])) {
      |                  ~~~~~~~~~^~~
src/fzf.c: In function 'trailing_whitespaces':
src/fzf.c:94:27: error: array subscript has type 'char' [-Werror=char-subscripts]
   94 |     if (!isspace(str->data[i])) {
      |                  ~~~~~~~~~^~~
src/fzf.c: In function '__prefix_match':
src/fzf.c:852:29: error: array subscript has type 'char' [-Werror=char-subscripts]
  852 |   if (!isspace(pattern->data[0])) {
      |                ~~~~~~~~~~~~~^~~
src/fzf.c: In function '__suffix_match':
src/fzf.c:894:49: error: array subscript has type 'char' [-Werror=char-subscripts]
  894 |   if (len_pattern == 0 || !isspace(pattern->data[len_pattern - 1])) {
      |                                    ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make: *** [Makefile:19: build/libfzf.dll] Error 1

Using the same version of gcc from msys2 mingw64 doesn't have this issue.

[Conni2461]

Had to check the specification but yeah you are right.

Thanks :)