Fix compilation errors on VS2017 for x86

MSVC errors without this patch:
tinyexpr.c(161): error C2099: initializer is not a constant
tinyexpr.c(161): warning C4047: 'initializing': 'const void *' differs in levels of indirection from 'int'
tinyexpr.c(167): error C2099: initializer is not a constant
tinyexpr.c(167): warning C4047: 'initializing': 'const void *' differs in levels of indirection from 'int'

tinyexpr.c

@@ -148,6 +148,8 @@ static double ncr(double n, double r) {
     return result;
 }
 static double npr(double n, double r) {return ncr(n, r) * fac(r);}
+static double te_ceil(double x) {return ceil(x);}
+static double te_floor(double x) {return floor(x);}
 
 static const te_variable functions[] = {
     /* must be in alphabetical order */
@@ -156,13 +158,13 @@ static const te_variable functions[] = {
     {"asin", asin,    TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"atan", atan,    TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"atan2", atan2,  TE_FUNCTION2 | TE_FLAG_PURE, 0},
-    {"ceil", ceil,    TE_FUNCTION1 | TE_FLAG_PURE, 0},
+    {"ceil", te_ceil, TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"cos", cos,      TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"cosh", cosh,    TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"e", e,          TE_FUNCTION0 | TE_FLAG_PURE, 0},
     {"exp", exp,      TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"fac", fac,      TE_FUNCTION1 | TE_FLAG_PURE, 0},
-    {"floor", floor,  TE_FUNCTION1 | TE_FLAG_PURE, 0},
+    {"floor",te_floor,TE_FUNCTION1 | TE_FLAG_PURE, 0},
     {"ln", log,       TE_FUNCTION1 | TE_FLAG_PURE, 0},
 #ifdef TE_NAT_LOG
     {"log", log,      TE_FUNCTION1 | TE_FLAG_PURE, 0},