cli: fix undefined alloca() on CYGWIN

I tried to build my libgit2-1.9.0 package for CYGWIN but I got an error. This message appears when compiling:

[199/671] Building C object src/cli/CMakeFiles/git2_cli.dir/opt.c.o
libgit2-1.9.0/src/cli/opt.c: In function ‘cli_opt_parse’:
libgit2-1.9.0/src/cli/opt.c:564:23: warning: implicit declaration of function ‘alloca’; did you mean ‘malloc’? [-Wimplicit-function-declaration]
  564 |         given_specs = alloca(sizeof(const cli_opt_spec *) * (args_len + 1));
      |                       ^~~~~~
      |                       malloc

and later the linker emits this error message:

[668/671] Linking C executable git2.exe
FAILED: git2.exe
/usr/x86_64-pc-cygwin/bin/ld: src/cli/CMakeFiles/git2_cli.dir/opt.c.o: in function `cli_opt_parse':
/usr/src/debug/libgit2-1.9.0-1/src/cli/opt.c:564:(.text+0xce3): undefined reference to `alloca'
collect2: error: ld returned 1 exit status

The error is fixed by adding alloca.h to included headers.
Hopefully, opt.c already allows to add alloca.h for some platforms, so I just added an additional test for the preprocessor for checking if the target is CYGWIN.

src/cli/opt.c

@@ -19,7 +19,7 @@
 #include <limits.h>
 #include <assert.h>
 
-#if defined(__sun) || defined(__illumos__)
+#if defined(__sun) || defined(__illumos__) || defined(__CYGWIN__)
 # include <alloca.h>
 #endif