package/libest: doesn't support libressl

Fix the following build failure with libressl raised since the addition
of the package in commit f6f0e1e58168dfa9194db2e073efd20a98c15728:

In file included from est.c:28:
est.h:27:10: fatal error: openssl/srp.h: No such file or directory
   27 | #include <openssl/srp.h>
      |          ^~~~~~~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/0dd755effbdddfa1b578fec9f1b1c9366b6822bc

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/libest/Config.in

@@ -6,6 +6,7 @@ config BR2_PACKAGE_LIBEST
 	depends on !BR2_STATIC_LIBS # libexecinfo or glibc
 	select BR2_PACKAGE_LIBEXECINFO if !BR2_TOOLCHAIN_USES_GLIBC
 	select BR2_PACKAGE_OPENSSL
+	select BR2_PACKAGE_OPENSSL_FORCE_LIBOPENSSL
 	help
 	  libest is a C implementation of RFC 7030 (Enrollment over
 	  Secure Transport).