Add missing inline attributes to avoid linker issues.

The `xoshiro<2>::do_jump` specializations need some `inline` attribute,
otherwise including dqrng from multiple compilation units causes linker
errors due to symbols being defined multiple times.

inst/include/xoshiro.h

@@ -104,7 +104,7 @@ class xoshiro {
 };
 
 template<>
-void xoshiro<2>::do_jump(std::array<result_type, 2> JUMP) {
+inline void xoshiro<2>::do_jump(std::array<result_type, 2> JUMP) {
     uint64_t s0 = 0;
     uint64_t s1 = 0;
     for(unsigned int i = 0; i < sizeof JUMP / sizeof JUMP.begin(); i++)
@@ -121,7 +121,7 @@ void xoshiro<2>::do_jump(std::array<result_type, 2> JUMP) {
 }
 
 template<>
-void xoshiro<4>::do_jump(std::array<result_type, 4> JUMP) {
+inline void xoshiro<4>::do_jump(std::array<result_type, 4> JUMP) {
     uint64_t s0 = 0;
     uint64_t s1 = 0;
     uint64_t s2 = 0;