cmake: emit warning if X11_Xi or X11_XTest not found

at least on Debian, a bullseye installation, the X11 development files
may be installed without the libxi-dev or the libxtest-dev packages.
this leads to the autotype shared library libkeepassxc-autotype-xcb.so
not being built without any complaint from cmake.

this commit makes cmake emit warning messages that shall hint anyone
building themselves that autotype will not work without these libs.

src/autotype/CMakeLists.txt

@@ -8,6 +8,16 @@ if(WITH_XC_AUTOTYPE)
             add_feature_info(Qt5X11Extras Qt5X11Extras_FOUND "The Qt5X11Extras library is required for auto-type")
         endif()
 
+        # an error will be emitted by find_package above if X11 (or Qt5X11Extras) dev files were not found.
+        # X11 possibly is installed and found while X11_Xi and/or X11_XTest are not, so we emit a warning.
+        if(NOT X11_Xi_FOUND)
+            message(WARNING "The X11 Xi Protocol library is required for auto-type (try libxi-dev package)")
+        endif()
+
+        if(NOT X11_XTest_FOUND)
+            message(WARNING "The X11 XTEST Protocol library is required for auto-type (try libxtst-dev package)")
+        endif()
+
         if(X11_FOUND AND X11_Xi_FOUND AND X11_XTest_FOUND AND Qt5X11Extras_FOUND)
             add_subdirectory(xcb)
         endif()