Add strerror to cstring/string.h

And remove duplicate definitions

Author: lcartey

cpp/common/test/includes/standard-library/cstring

@@ -15,6 +15,7 @@ using ::strcmp;
 using ::strcoll;
 using ::strcpy;
 using ::strcspn;
+using ::strerror;
 using ::strlen;
 using ::strncat;
 using ::strncmp;

cpp/common/test/includes/standard-library/string.h

@@ -35,14 +35,13 @@ const char *strstr(const char *str1, const char *str2);
 char *strstr(char *str1, const char *str2);
 
 char *strtok(char *str, const char *delimiters);
+char *strerror(int errnum);
 
-char *strdup (const char *);
+char *strdup(const char *);
 
 void *memcpy(void *dest, const void *src, size_t count);
 void *memset(void *dest, int ch, size_t count);
 void *memmove(void *dest, const void *src, size_t count);
 int memcmp(const void *lhs, const void *rhs, size_t count);
 
-size_t strlen(const char *str);
-
 #endif // _GHLIBCPP_STRINGH