Minor compilation fix

[eddelbuettel]

I looked over how RcppSMC 0.2.4 fared at CRAN and noticed that Solaris is unhappy with a sole use of an unqualified pow():

/opt/csw/bin/g++ -std=gnu++14 -I"/home/ripley/R/gcc/include" -DNDEBUG -I../inst/include -I'/home/ripley/R/Lib32/Rcpp/include' -I'/home/ripley/R/Lib32/RcppArmadillo/include' -I/opt/csw/include -I/usr/local/include  -fPIC  -O2  -c RcppExports.cpp -o RcppExports.o
In file included from ../inst/include/RcppSMC.h:25:0,
                 from RcppExports.cpp:4:
../inst/include/conditionalSampler.h: In member function ‘std::ostream& smc::conditionalSampler<Space, Params>::StreamParticles(std::ostream&, int) const’:
../inst/include/conditionalSampler.h:630:41: error: call of overloaded ‘pow(int, int&)’ is ambiguous
         int roundDigits = pow(10, digits);
                                         ^
In file included from /usr/include/math.h:15:0,
                 from /opt/csw/include/c++/5.2.0/cmath:44,
                 from /home/ripley/R/Lib32/Rcpp/include/Rcpp/platform/compiler.h:100,
                 from /home/ripley/R/Lib32/Rcpp/include/Rcpp/r/headers.h:66,
                 from /home/ripley/R/Lib32/Rcpp/include/RcppCommon.h:30,
                 from /home/ripley/R/Lib32/RcppArmadillo/include/RcppArmadilloForward.h:26,
                 from /home/ripley/R/Lib32/RcppArmadillo/include/RcppArmadillo.h:31,
                 from ../inst/include/RcppSMC.h:18,
                 from RcppExports.cpp:4:
/opt/csw/lib/gcc/i386-pc-solaris2.10/5.2.0/include-fixed/iso/math_iso.h:204:21: note: candidate: long double std::pow(long double, long double)
  inline long double pow(long double __X, long double __Y) { return
                     ^
/opt/csw/lib/gcc/i386-pc-solaris2.10/5.2.0/include-fixed/iso/math_iso.h:171:15: note: candidate: float std::pow(float, float)
  inline float pow(float __X, float __Y) { return __powf(__X, __Y); }
               ^
/opt/csw/lib/gcc/i386-pc-solaris2.10/5.2.0/include-fixed/iso/math_iso.h:72:15: note: candidate: double std::pow(double, double)
 extern double pow __P((double, double));
               ^
*** Error code 1
make: Fatal error: Command failed for target `RcppExports.o'

I have the fix ready, and even tested it via RHub and its Solaris builder -- it just needs a std::pow as usual.

I will roll up a PR and suggest we do 0.2.5