DPP: Fix a regression in non-DPP, non-OpenSSL builds

DPP: Fix a regression in non-DPP, non-OpenSSL builds

Inclusion of common/dpp.h into hostapd/main.c brought in an undesired
unconditional dependency on OpenSSL header files even for builds where
DPP is not enabled. Fix this by making the dpp.h contents, and in
particular the inclusion of openssl/x509.h, conditional on CONFIG_DPP.

Fixes: 87d8435cf9fd ("DPP: Common configurator/bootstrapping data management")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>