Partitions: Fix minor header include issues

tfm_veneers.h is used by Library Mode only.
This patch fixes some include issue where tfm_veneer.h is included
for both Mode.

Change-Id: I87830222e7fec0098ed77f43006639dba3937a44
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/cmake/install.cmake b/cmake/install.cmake
index 653dcc4..2af1476 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -55,7 +55,7 @@
                         ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox.h
                         ${INTERFACE_INC_DIR}/multi_core/tfm_mailbox_config.h
             DESTINATION ${INSTALL_INTERFACE_INC_DIR})
-else()
+elseif (NOT TFM_PSA_API)
     install(FILES       ${CMAKE_BINARY_DIR}/generated/interface/include/tfm_veneers.h
             DESTINATION ${INSTALL_INTERFACE_INC_DIR}/tfm/veneers)
 endif()