This CL adds srtp.h to the list of SSLHeaderFiles, in order to move it
from ssl_h_files to crypto_h_files. The header file only includes an
inclusion of ssl.h. ssl_h_files can depend on crypt_h_files but not the
other way around.

Change-Id: If7410624a8b2bbbd5afb7f66ec6f491968faf24e
Reviewed-on: https://boringssl-review.googlesource.com/c/32505
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>