build: Fix openssl build when not using the bundled library.

Otherwise, with
cmake -DCMAKE_BUILD_TYPE=Debug  -DUSE_BUNDLED_OPENSSL=OFF ..
one gets

```
[ 96%] Linking CXX executable csysdig
[ 97%] Linking CXX executable sysdig
/usr/bin/ld: ../libsinsp/libsinsp.a(k8s_handler.cpp.o): undefined reference to symbol 'SSL_CTX_use_PrivateKey_file'
/usr/lib/libssl.so.1.0.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [userspace/sysdig/CMakeFiles/csysdig.dir/build.make:131: userspace/sysdig/csysdig] Error 1
make[1]: *** [CMakeFiles/Makefile2:275: userspace/sysdig/CMakeFiles/csysdig.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
/usr/bin/ld: ../libsinsp/libsinsp.a(k8s_handler.cpp.o): undefined reference to symbol 'SSL_CTX_use_PrivateKey_file'
/usr/lib/libssl.so.1.0.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [userspace/sysdig/CMakeFiles/sysdig.dir/build.make:129: userspace/sysdig/sysdig] Error 1
make[1]: *** [CMakeFiles/Makefile2:323: userspace/sysdig/CMakeFiles/sysdig.dir/all] Error 2
make: *** [Makefile:139: all] Error 2
```

This is because linking to libssl and libcrypto is not done (after
find_package in CMakeLists.txt) when using
the system libraries.

sysdig-CLA-1.0-signed-off-by: Raghavendra Prabhu <me@rdprabhu.com>

userspace/libsinsp/CMakeLists.txt

@@ -94,12 +94,19 @@ if(NOT WIN32)
 			"${JQ_LIB}"
 			"${B64_LIB}"
 			"${CURL_LIBRARIES}"
-			"${OPENSSL_LIBRARY_SSL}"
-			"${OPENSSL_LIBRARY_CRYPTO}"
 			rt
 			anl)
 	endif()
 
+        if(USE_BUNDLED_OPENSSL)
+		target_link_libraries(sinsp
+			"${OPENSSL_LIBRARY_SSL}"
+			"${OPENSSL_LIBRARY_CRYPTO}")
+        else()
+		target_link_libraries(sinsp
+                    "${OPENSSL_LIBRARIES}")
+        endif()
+
 	target_link_libraries(sinsp
 		"${LUAJIT_LIB}"
 		dl