Conanfile: Add protobuf-nanopb library to cpp_info.libs (#605)

Without this, there will be no links against the nanopb library when
nanopb is integrated by conan
diff --git a/conanfile.py b/conanfile.py
index 515e560..d4a44fb 100644
--- a/conanfile.py
+++ b/conanfile.py
@@ -30,3 +30,4 @@
     def package_info(self):
         self.cpp_info.includedirs = ["include"]
         self.cpp_info.libdirs = ["lib"]
+        self.cpp_info.libs = ["protobuf-nanopb"]