Remove some unnecessary recursive includes from the internal apps.h header,
and include bn.h in those C files that need bignum functionality.

apps/dsa.c

diff --git a/apps/dsa.c b/apps/dsa.c
index e9de3a3..b6f0ed3 100644
--- a/apps/dsa.c
+++ b/apps/dsa.c
@@ -68,6 +68,7 @@
 #include <openssl/evp.h>
 #include <openssl/x509.h>
 #include <openssl/pem.h>
+#include <openssl/bn.h>
 
 #undef PROG
 #define PROG	dsa_main