Fix compiler warnings.

Changed finds that use a string with a single character ("X"), to 'X'.

Remove using statements from header file.

Add a couple of missing includes.

Changed a couple of functions that take a string to const string& to
avoid extra copies.

Test: No warnings when compiled, unit tests pass.
Change-Id: I218ee0e6b6c7fb1c79d1337c1ce3d6f1e318a3cd
diff --git a/libbpf_android/Loader.cpp b/libbpf_android/Loader.cpp
index 7687cb1..d02ed87 100644
--- a/libbpf_android/Loader.cpp
+++ b/libbpf_android/Loader.cpp
@@ -34,6 +34,7 @@
 #include <fstream>
 #include <iostream>
 #include <string>
+#include <vector>
 
 #include <android-base/strings.h>
 
@@ -45,6 +46,7 @@
 using android::base::StartsWith;
 using std::ifstream;
 using std::ios;
+using std::string;
 using std::vector;
 
 namespace android {
@@ -498,7 +500,7 @@
     }
 }
 
-static int loadCodeSections(const char* elfPath, vector<codeSection>& cs, string license) {
+static int loadCodeSections(const char* elfPath, vector<codeSection>& cs, const string& license) {
     int ret, fd, kvers;
 
     if ((kvers = getMachineKvers()) < 0) return -1;