Move utility functions trimfront/back to tccpp.c

These are used in `libtcc.c` now and cannot remain in `tccpe.c`

tcc.h

@@ -1263,6 +1263,8 @@ ST_FUNC void preprocess_delete(void);
 ST_FUNC int tcc_preprocess(TCCState *s1);
 ST_FUNC void skip(int c);
 ST_FUNC NORETURN void expect(const char *msg);
+ST_FUNC char *trimfront(char *p);
+ST_FUNC char *trimback(char *a, char *e);
 
 /* ------------ tccgen.c ------------ */