security/i2pd: add lowest boost-libs version required

Build of i2pd-2.38.0 may break with obscure error unless
devel/boost-libs updated to 1.72.0_5 at least.
Make it complain with clear message instead in such case.

This unbreaks build, so PORTREVISION not bumped.

security/i2pd/Makefile

@@ -8,6 +8,7 @@ COMMENT=	C++ implementation of I2P client
 LICENSE=	BSD3CLAUSE
 LICENSE_FILE=	${WRKSRC}/LICENSE
 
+BUILD_DEPENDS=	boost-libs>=1.72.0_5:devel/boost-libs
 LIB_DEPENDS=	libboost_thread.so:devel/boost-libs
 
 USE_GITHUB=	yes