net/uvgrtp: pass down BUILD_SHARED_LIBS to fix the port's build

By specifying add_library(... SHARED) we were instructing CMake
to build all but one objects in PIC mode, in turn causing the
infamous "relocation R_X86_64_32S cannot be used against local
symbol" error upon linking the program. Mend this by using the
documented and cleaner BUILD_SHARED_LIBS approach. While here,
fix an IF block open/close arguments mismatch CMake warning.

Reported by: pkg-fallout