config/kernel-inode-times: initialize timespec

Usage of this variable uninitialized triggers -Werror,-Wuninitialized
when compiled under clang for linux kernel 5.6, leading the build system
to believe that the function is not declared.

This commit initializes the variable to suppress the warning and fix the
build for kernel 5.6 with clang.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Hiếu Lê <leorize+oss@disroot.org>
Closes openzfs#10279
Closes openzfs#10281

config/kernel-inode-times.m4

@@ -10,6 +10,7 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_INODE_TIMES], [
 		struct timespec64 ts;
 		struct inode ip;
 
+		memset(&ts, 0, sizeof(ts));
 		ts = timestamp_truncate(ts, &ip);
 	])