Fix build w/o OpenSSL by w/ MbedTLS

In 78fa397 from
libevent#1241 (comment) a
new "if MBEDTLS" block was added to include/include.am. This is not
parsed by automake as it isn't on the first column, resulting in a
subsequent error in the build.

From [1]:

> The if, else, and endif statements should not be indented, i.e., start on column one.

  [1]: https://www.gnu.org/software/automake/manual/html_node/Usage-of-Conditionals.html

Fixes: libevent#1251
Fixes: google/oss-fuzz#7149

include/include.am

@@ -36,9 +36,9 @@ EVENT2_EXPORT = \
 if OPENSSL
 EVENT2_EXPORT += include/event2/bufferevent_ssl.h
 else
-    if MBEDTLS
-    EVENT2_EXPORT += include/event2/bufferevent_ssl.h
-    endif
+if MBEDTLS
+EVENT2_EXPORT += include/event2/bufferevent_ssl.h
+endif
 endif
 
 ## Without the nobase_ prefixing, Automake would strip "include/event2/" from