Make sure `OPENSSL_INCLUDE_DIR` is defined before using it in `INCLUDE_DIRECTORIES`

OPENSSL_INCLUDE_DIR is defined by `FIND_PACKAGE` so it might be empty if
it's used to soon in the file.

CMakeLists.txt

@@ -8,8 +8,7 @@ PROJECT(picoquic)
 SET(CMAKE_C_FLAGS "-std=c99 -Wall -O2 -g ${CC_WARNING_FLAGS} ${CMAKE_C_FLAGS}")
 
 INCLUDE_DIRECTORIES(picoquic picoquictest ../picotls/include
-    ${PICOTLS_INCLUDE_DIR}
-    ${OPENSSL_INCLUDE_DIR})
+    ${PICOTLS_INCLUDE_DIR})
 
 SET(PICOQUIC_LIBRARY_FILES
     picoquic/fnv1a.c
@@ -58,6 +57,7 @@ MESSAGE("root: ${OPENSSL_ROOT_DIR}")
 MESSAGE("OpenSSL_VERSION: ${OPENSSL_VERSION}")
 MESSAGE("OpenSSL_INCLUDE_DIR: ${OPENSSL_INCLUDE_DIR}")
 MESSAGE("OpenSSL_LIBRARIES: ${OPENSSL_LIBRARIES}")
+INCLUDE_DIRECTORIES(${OPENSSL_INCLUDE_DIR})
 
 ADD_LIBRARY(picoquic-core
     ${PICOQUIC_LIBRARY_FILES}