Android: link libdl into rnp's OpenSSL feature probe

[jolavillette]

build_librnp fails while rnp configures itself:

ld.lld: error: undefined symbol: dlopen
>>> referenced by dso_dlfcn.c
>>>               dso_dlfcn.o:(dlfcn_load) in archive .../sysroot/usr/lib/libcrypto.a

CMake Error at cmake/Modules/FindOpenSSLFeatures.cmake:149 (message):
  Error building findopensslfeatures

rnp compiles a small findopensslfeatures helper and runs it to enumerate the
OpenSSL features. When cross-compiling it links that helper statically, so it
does not need the Android dynamic linker:

if(CMAKE_CROSSCOMPILING_EMULATOR)
  target_link_options(findopensslfeatures PRIVATE -static)
endif(CMAKE_CROSSCOMPILING_EMULATOR)

but OpenSSL 1.1.1's libcrypto.a pulls in dso_dlfcn.o, which references the
dl* API, and CMake's FindOpenSSL does not add ${CMAKE_DL_LIBS} to the
OpenSSL::Crypto interface here. The link line ends up as

clang -O3 -DNDEBUG -static ...o -o findopensslfeatures  .../libcrypto.a -pthread

with nothing providing dlopen, dlsym, dlclose and dlerror.

The NDK does ship a static libdl.a defining those four for real, so it is
enough to link it. It is added as a linked library rather than through
CMAKE_EXE_LINKER_FLAGS, because CMake places that variable before the
objects, where a static linker has no undefined symbol to resolve yet and drops
the archive silently.

This is arguably rnp's bug — their -static path looks untested against a
static OpenSSL 1.1.1 — so the patch stays a sed next to the existing ones, and
should be dropped once rnp handles it.

Verified

build_librnp completes and installs librnp.a in the sysroot, and the whole
AAR now builds: 42 MB, jni/arm64-v8a/libretroshare.so exporting JNI_OnLoad,
the four org.retroshare.service classes in classes.jar. Ubuntu 24.04, NDK
29.0.14206865, API level 24, arm64-v8a, on top of #373 and #375.