Add external c declaration for c++

To make sure c++ name mangling works correctly c code should be noted "extern"

Signed-off-by: Norbert Pocs <npocs@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>

include/libssh/string.h

@@ -22,6 +22,10 @@
 #define STRING_H_
 #include "libssh/priv.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* must be 32 bits number + immediately our data */
 #ifdef _MSC_VER
 #pragma pack(1)
@@ -38,4 +42,8 @@ __attribute__ ((packed))
 #endif
 ;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* STRING_H_ */