openssl: Do not require the rtc_ssl_root variable

If the includes are in /usr/include, adding this path to the include path
may break resolution of internal compiler headers.
If the variable is required, I would expect users to already have it set
properly.

Bug: None
Change-Id: I1d86776da4ae516aba99c58ecee1135dcd27aec8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/359240
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42763}

rtc_base/BUILD.gn

@@ -15,9 +15,9 @@ if (is_android) {
 
 if (!rtc_build_ssl) {
   config("external_ssl_library") {
-    assert(rtc_ssl_root != "",
-           "You must specify rtc_ssl_root when rtc_build_ssl==0.")
-    include_dirs = [ rtc_ssl_root ]
+    if (rtc_ssl_root != "") {
+      include_dirs = [ rtc_ssl_root ]
+    }
     libs = [
       "crypto",
       "ssl",