Link libpcrecpp for USE_PCRE.

megacmdutils.cpp includes <pcrecpp.h> and uses pcrecpp::RE, which lives
in libpcrecpp, not libpcre. Upstream marks this path "UNTESTED".

diff --git a/build/cmake/modules/megacmd_libraries.cmake b/build/cmake/modules/megacmd_libraries.cmake
index aa3c2d7..59f9acb 100644
--- a/build/cmake/modules/megacmd_libraries.cmake
+++ b/build/cmake/modules/megacmd_libraries.cmake
@@ -16,7 +16,7 @@ macro(load_megacmdserver_libraries)
         find_package(PkgConfig REQUIRED) # For libraries loaded using pkg-config
 
         if(USE_PCRE) #TODO: UNTESTED!
-            pkg_check_modules(pcre REQUIRED IMPORTED_TARGET libpcre)
+            pkg_check_modules(pcre REQUIRED IMPORTED_TARGET libpcrecpp)
             target_link_libraries(LMegacmdServer PRIVATE PkgConfig::pcre)
             set(USE_PCRE 1)
         endif()