fix include names

cppcheck isn't smart enough to recognize these are c++ headers, not c
headers.  Considering we're already inconsistent about our naming, it's
easier to just be consistent, and move the last few files to use .hpp
instead of .h.

Tested:
Code builds, no changes.

Signed-off-by: Ed Tanous <ed@tanous.net>
Change-Id: Ic348d695f8527fa4a0ded53f433e1558c319db40
diff --git a/src/security_headers_middleware_test.cpp b/src/security_headers_middleware_test.cpp
index 500839a..f5cd3d5 100644
--- a/src/security_headers_middleware_test.cpp
+++ b/src/security_headers_middleware_test.cpp
@@ -1,5 +1,4 @@
-#include <app.h>
-
+#include <app.hpp>
 #include <security_headers_middleware.hpp>
 
 #include <gmock/gmock.h>