AP_SpdHgtControl: standardize inclusion of libaries headers

This commit changes the way libraries headers are included in source files:

 - If the header is in the same directory the source belongs to, so the
 notation '#include ""' is used with the path relative to the directory
 containing the source.

 - If the header is outside the directory containing the source, then we use
 the notation '#include <>' with the path relative to libraries folder.

Some of the advantages of such approach:

 - Only one search path for libraries headers.

 - OSs like Windows may have a better lookup time.

libraries/AP_SpdHgtControl/AP_SpdHgtControl.h

@@ -13,9 +13,9 @@
 #ifndef AP_SPDHGTCONTROL_H
 #define AP_SPDHGTCONTROL_H
 
-#include <AP_Common.h>
-#include <AP_Param.h>
-#include <DataFlash.h>
+#include <AP_Common/AP_Common.h>
+#include <AP_Param/AP_Param.h>
+#include <DataFlash/DataFlash.h>
 
 class AP_SpdHgtControl {
 public: