CMakeLists.txt: remove vendored tinydtls build path

Do not write the absolute build path into libcoapTargets.cmake. The
libcoapTargets.cmake is supposed to be packaged and installed by
distros. An absolute path referring to the build directory looks
therefore wrong.
Some build frameworks like Yocto have quality assurance check which
handle these issues as a warning or as an error. Example:

WARNING: libcoap-4.3.4-r0 do_package_qa: QA Issue:
  File /usr/lib/cmake/libcoap/libcoapTargets.cmake in package
  libcoap-dev contains reference to TMPDIR [buildpaths]

Declaring this header files as build interface looks like the right fix.
In case of a bundled tinytls library the header files should be handled
like the header files from libcoap itself: Either in the build folder of
libcoap or in /usr/include as declared one line above.

CMakeLists.txt

@@ -829,7 +829,7 @@ target_include_directories(
          $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include/>
          $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}/include/>
          $<INSTALL_INTERFACE:include/>
-         $<$<AND:$<BOOL:${COAP_WITH_LIBTINYDTLS}>,$<BOOL:${USE_VENDORED_TINYDTLS}>>:${CMAKE_BINARY_DIR}/include/tinydtls>
+         $<BUILD_INTERFACE:$<$<AND:$<BOOL:${COAP_WITH_LIBTINYDTLS}>,$<BOOL:${USE_VENDORED_TINYDTLS}>>:${CMAKE_BINARY_DIR}/include/tinydtls>>
          $<$<BOOL:${COAP_WITH_LIBGNUTLS}>:${GNUTLS_INCLUDE_DIR}>
          $<$<BOOL:${COAP_WITH_LIBMBEDTLS}>:${MBEDTLS_INCLUDE_DIRS}>
          $<$<BOOL:${COAP_WITH_LIBWOLFSSL}>:${WOLFSSL_INCLUDE_DIR}>)