Add missing imports of conan recipes

recipes/ls-std/2021_1_0/conanfile.py

@@ -1,3 +1,6 @@
+from conans import ConanFile, CMake, tools
+
+
 class LsStdConan(ConanFile):
     name = "ls-std"
     version = "2021.1.0"

recipes/ls-std/2021_2_0/conanfile.py

@@ -1,3 +1,6 @@
+from conans import ConanFile, CMake, tools
+
+
 class LsStdConan(ConanFile):
     name = "ls-std"
     version = "2021.2.0"