Unhardcode "lsblk" path

On many systems, lsblk might not be available exactly on /usr/bin. Since
popen() respects $PATH, it makes sense to avoid hardcoding this path.

--- sdFormatLinux-0.2.0.orig/source/blockdev.cpp
+++ sdFormatLinux-0.2.0/source/blockdev.cpp
@@ -21,8 +21,8 @@
 static int checkDevice(const char *const path)
 {
 	int res = EINVAL; // By default assume the given path is not a suitable device.
-	char cmd[64] = "/usr/bin/lsblk -dnr -oTYPE,HOTPLUG,PHY-SEC ";
-	strncpy(&cmd[43], path, sizeof(cmd) - 43);
+	char cmd[64] = "lsblk -dnr -oTYPE,HOTPLUG,PHY-SEC ";
+	strncpy(&cmd[34], path, sizeof(cmd) - 34);
 	cmd[sizeof(cmd) - 1] = '\0';
 	FILE *const p = ::popen(cmd, "r");
 	if(p == nullptr)