fastboot: fix windows sdk build (don't do f2fs dyn load on windows)

Only do f2fs dynamic loading on linux.
Adds a stub for windows builds.

Change-Id: I7bbaaa1b2ff5992709d904b7ace40ae263d32922
Signed-off-by: JP Abgrall <jpa@google.com>
diff --git a/fastboot/fs.c b/fastboot/fs.c
index 11e391b..8a15e6f 100644
--- a/fastboot/fs.c
+++ b/fastboot/fs.c
@@ -29,11 +29,12 @@
     return 0;
 }
 
-int generate_f2fs_image(int fd, long long partSize)
+#ifdef USE_F2FS
+static int generate_f2fs_image(int fd, long long partSize)
 {
-    make_f2fs_sparse_fd(fd, partSize, NULL, NULL);
-    return 0;
+    return make_f2fs_sparse_fd(fd, partSize, NULL, NULL);
 }
+#endif
 
 static const struct fs_generator {
 
@@ -42,7 +43,9 @@
 
 } generators[] = {
     { "ext4", generate_ext4_image},
+#ifdef USE_F2FS
     { "f2fs", generate_f2fs_image},
+#endif
 };
 
 const struct fs_generator* fs_get_generator(const char *fs_type)